Python Graphical User Interface

A PyQT GUI has been created to make it easier to keep track of the loaded NeXus files and the results of any subsequent analysis. It is invoked from the command line by:

> nexpy


This assumes that the Python ‘bin’ directory is in your standard shell path.


The illustration shows the main features of the GUI:

1) Tree Pane

This contains the tree structure of NeXus or HDF5 files opened in the File menu, non-NeXus files that have been imported and converted into the NeXus format using one of the NeXus readers, and NXroot, NXentry, or NXdata groups added from the shell. Various actions on the data can be performed by right-clicking a tree item, include plotting, renaming, fitting and deleting the data.

2) Plot Pane

This contains plots produced by (a) the Data:Plot Data menu item, which operates on the NeXus data selected in the tree, (b) double-clicking on NeXus data in the tree, or (c) using NeXus data plot methods from the shell. If an NXdata, NXmonitor, or NXlog group is plotted, the rank, dimensions, and plotting axes are determined automatically. If the rank of the data is greater than two, a two-dimensional slice is extracted from the data. The GUI allows the selection of alternative slices using one of the axis panels (see below). If an NXfield is selected, the axes can be chosen from other NXfields in the same group. It is possible to open other plot windows and switch between them using the Window menu or keyboard shortcuts (see below).

3) Shell Pane

This is an IPython shell, with NeXpy already imported (as * so no prefixes are necessary), along with NumPy (as ‘np’), Pyplot (as ‘plt’), and Matplotlib (as ‘mpl’). Any assignments to items in the tree pane are automatically reflected in the tree pane, and new NXroot or NXentry objects can be added to the tree from the IPython shell. NeXus data plots commands from the shell will appear in the plot pane, and Matplotlib commands can be used to modify the plot characteristics. The shell has enhanced features such as autocompletion of NeXus dictionaries and attributes and tooltips containing module docstrings when you open the module parentheses.

4) Axis Panels

The tabbed panels below the plot can be used to modify the plots. The number of panels depends on the rank of the original data. The ‘signal’, ‘x’ and ‘y’ panels have text boxes and sliders for adjusting the plotting limits. For rank two or more, a projection panel allows the plotting of projections along different directions, using the current axis limits. For ranks greater than two, a ‘z’ panel allows the other dimensions to be varied. Finally, the ‘options’ panel provides access to the a number of tools for modifying the plot and saving/exporting the plotted data.

5) Status Bar

The values and attributes of the currently selected item in the tree are displayed in the status bar.

6) Tooltips

The NeXus tree structure of an item in the tree pane will be displayed as a tooltip when the cursor hovers over it.

NeXpy Menu Bar

File Menu


Creates a new workspace in the tree.


Opens a new NeXus file as read-only. It is possible unlock the file to allow modifications to the file (see below).


It is possible to open a file in directly read/write mode using the keyboard shortcut Ctrl+Shift+O (⌘+⇧+O on a Mac). Note that any changes to the file tree, using either the shell or GUI commands, will be automatically updated in the file.

Open Recent…

Allows one of the 20 most recently opened or saved files to be opened. Hovering over one of the files in the list shows its absolute path.

Open Image…

Opens an image file and imports the image and any stored metadata into an NXdata group within a root tree item, called images. This will read TIFF and CBF files if FabIO is installed. JPEG, PNG, and GIF files are imported using Pillow. RGB(A) images contain three-dimensional arrays, including color (and transparency) layers, which can be displayed as two-dimensional images, with the y-axis inverted according to the usual image convention, using Plot RGB(A) Image.

Open Directory…

Opens all the HDF5 files stored in the selected directory. It does not reopen files already loaded into the tree.

Save as…

Saves the selected tree item to a new NeXus file.


Makes a copy of the selected item, leaving the original untouched. If any field in the original tree is too large to be stored in memory, its data is stored in an HDF5 core memory file until the tree is saved to a file.

Restore Session

Loads all the files that were open at the end of the previous NeXpy session.


When launching NeXpy, files from the previous session can be opened using the -r switch.


Reloads the NeXus file. This is useful if another application has modified the data since originally opening the file.


If an external process has modified the currently loaded file, the lock icon color is changed to red. If the file was previously unlocked, its mode is automatically changed to read-only when the modification is detected.

Reload All

Reloads all open NeXus files that have been modified by extermal processes.


Removes the root item from the NeXpy tree.


This will also remove the item with the same name from the shell. However, if it had previously been assigned to another variable with a different name, that variable will not be deleted.

Remove All

Removes all items from the NeXpy tree.

Collapse Tree

Collapses all expanded items in the tree.


Imports data from other formats. Some importers are provided with the NeXpy distribution, but others will be loaded from the user’s ~/.nexpy/readers directory.


Exports data to a NeXus file or, for one-dimensional data, to a multi-column ASCII file.

Lock File

Changes the file access mode to read-only. This will prevent further changes to the tree using either the GUI or the shell. Locked files are displayed with a padlock icon.

Unlock File

Changes the file access mode to read/write. If the root tree item has been saved to a file, any subsequent changes will automatically update the file, including deleting items.


Any changes to an unlocked file will be immediate and irreversible. If the file contains critical data, click the checkbox to create a backup, which can be restored later if necessary.

Show File Locks

If a Lock Directory has been defined, this shows a listing of all files, whose access is blocked by a current lock file.

See also

File Locking

Backup File

Creates a backup of the selected file. The backup is stored in the user’s home directory in ~/.nexpy/backups and may be restored if changes to the currently open file need to be reversed. Backups are saved for five days before being automatically deleted.

Restore Backup…

Restores the backup of this file. The user is prompted to confirm that the currently open file should be overwritten.


This only applies to backups created during the current session. Previously saved backups can be restored using the Manage Backups menu item.

Manage Backups…

Provides the ability to restore or delete an existing backup stored in ~/.nexpy/backups. Restoring the backup is equivalent to opening the existing backup file. It is necessary to save it to a new location to prevent its automatic deletion after five days.

Open Scratch File…

Saved projections and fits are stored in a scratch file called w0.nxs, which is stored in the user’s NeXpy directory, ~/.nexpy. This file is automatically opened when new data is saved, but this menu item allows it to be opened at any time.

Purge Scratch File…

Previously saved items can be manually removed from the scratch file when they are no longer needed. This menu item purges all the items in one go.

Close Scratch File…

Closes the scratch file.

Install Plugin

A directory containing a NeXpy plugin module can be installed either in the user’s NeXpy directory (~/.nexpy/plugins) or in the package directory if the user has the necessary privilege. The plugin menu is appended to the existing menus, but will be loaded in alphabetical order of the other plugins when NeXpy is restarted.


If a plugin of the same name exists in both directories, the user’s plugin is loaded.


With NeXpy v1.0.3, plugins can also be loaded directly from external packages, in which the plugin modules are declared using the nexpy.readers entry point.

See also

NeXpy Plugins

Remove Plugin

The selected NeXpy plugin module is removed from either the user’s NeXpy directory (~/.nexpy/plugins) or the package directory.

Restore Plugin

If a plugin is overwritten by installing another version, it is backed up in ~/.nexpy/backups). This allows the old version to be restored.

Edit Preferences

Open a dialog to set preferences for this session. It is also possible to save them as the default for subsequent sessions.

Data Menu

Plot Data

Plots the selected tree item in the plotting pane. If the selected item is not a valid NXdata, NXmonitor, or NXlog group, a plot dialog allows the user to specify axes with compatible dimensions to plot the data against.

Overplot Data

Overplots the selected tree item in the plotting pane. This only works on one-dimensional data.


The new plot is overlaid on the original plot using the same axis limits. If some of the new data lies outside the original plotting limits, the slider limits are increased to cover the expanded range.

Plot RGB(A) Image

Plots the selected tree item as an RGB(A) image. In such images, the fastest varying dimension, which should be of size 3 or 4, contains the RGB(A) values for each pixel. By convention, the first pixel is in the upper-left corner, rather than the lower-left.

View Data

Provides a tabular view of the selected item, whether it is a group or a field. All the metadata associated with the item, including any attributes, are displayed. For multidimensional data, a 10 x 10 slab of values is displayed, with spin boxes to select the slab offsets.

Add Data

Adds data to the selected tree item. If the selected item is a group, the added data can be a group or field. If the selected item is a field, the added data must be a field attribute.

When adding a field, the ‘Add Data’ dialog allows the name, value and data type to be specified. A dropdown menu can be used to enter field names that are defined by the NeXus standard, but the user is free to enter alternative names. The value field can be any valid Python expression, including NumPy functions such as np.linspace().

When adding a group, the ‘Add Data’ dialog allows the name and class of the group to be specified. A dropdown menu display can be used to enter one of the defined NeXus classes. Those above the dashed line are valid in the context of the selected tree item, but any of the other classes can also be selected.


If you click on the dropdown menus and hover over any item, a tooltip gives a description of its use.

Initialize Data

Adds a NeXus field to the selected tree item with the specified shape and data type, but without a predefined value. This is useful when creating large arrays that have to be entered as slabs. The shape box must contain a single integer, for a one-dimensional array, or a tuple (or list) of integers, for a multidimensional array. As with the ‘Add Data’ dialog, dropdown menus show the field names defined by the NeXus standard.

Rename Data

Renames the selected tree item. If the item is a group, its class can also be changed. Dropdown menus provide a list of valid group classes or field names defined by the NeXus standard.

Copy Data

Copies the selected tree item to a copy buffer.

Paste Data

Pastes the copy buffer to the selected group. If the selected group is in a file open with read/write access, all fields in the copy buffer are copied to the file. If the selected group is not currently stored in a file and any field in the copy buffer is too large to be stored in memory, its data is copied to an HDF5 memory-mapped file using the h5py copy module.

Paste As Link

Pastes a link to the copied node in the selected group. If the copied node and the selected group have different roots, the copied node is added to the group as an external link.


External links can only be modified through the parent file, which can be opened using the ‘Show Link’ menu item (see below).


The file containing the external link is referenced using the file path to the parent file. If the files are moved without preserving their relative file paths, the link will be broken.

Delete Data

Deletes the selected tree item.


If the item was assigned to another variable in the shell, that variable will not be deleted.


If the NeXus tree was loaded from a file with read/write access, the data will be immediately deleted from the file. This action is irreversible.

Show Link

Selects the field or group to which the selected item is linked, if it is an NXlink object, i.e., shown with a link icon. If the link is external, the linked file is automatically opened and the linked object is selected.

Set Signal

Sets the plottable signal either to the selected field or to any field within the selected group. A dialog box allows the user to specify axes with compatible dimensions to plot the data against.


The use of the ‘Add Data’ and ‘Set Signal’ menu items allows, in principle, an entire NeXus data tree to be constructed using menu calls.

Set Default

This sets the default attribute in the parent group to the currently selected group, i.e., if the selected group is an NXdata (NXentry) group, the attribute will be set in the parent NXentry (NXroot) group. The default attribute is used to identify the default data to be plotted.


When a NXdata group is set as the default, the parent NXentry group is also set as the default in the parent NXroot group provided one has not already been set. The default entry can be overridden.

Fit Data

Fits the selected tree item. This assumes that the selected item is a valid NXdata group. The menu item triggers a dialog box, which allows functions to be chosen and parameters to be initialized before calling a non-linear least-squares fitting module.

See also

Fitting NeXus Data.

Window Menu

Show Tree

Brings the tree view to the front and give it keyboard focus.


This has the keyboard shortcut of Ctrl+Shift+T (⌘+⇧+T on a Mac).

Show IPython Shell

Brings the shell to the front and give it keyboard focus.


This has the keyboard shortcut of Ctrl+Shift+I (⌘+⇧+I on a Mac).

Show Log File

Opens a text window displaying the NeXpy log file(s). These files, which are stored in ~/.nexpy/nexpy.log, ~/.nexpy/nexpy.log.1, etc., records operations on the tree items, as well as comprehensive tracebacks of exceptions in both the GUI and the IPython shell. Only one-line summaries are displayed in the shell to improve readability.


The log files contain ANSI markup to colorize the text, which can be rendered in the terminal using less -r.

Show Script Editor

Shows the script editor. If multiple scripts are open, they are displayed as tabs in a single window. If no scripts are open, this will open a new script.

Show Customize Panel

This opens a panel for the currently active plotting window that allows aspects of the plot, such as titles, axis labels, aspect ratios, skew angles, marker and line colors, and legends to be customized. All the open panels are displayed as tabs in a single window.



This is equivalent to clicking the Edit button in the Options Tab.

Show Limits Panel

This opens a panel for the currently active plotting window that allows the axes and axis limits of the currently active plot to be changed, as well as the plot size on the screen. All the panels are displayed as tabs in a single window, with the option of copying and values from one tab to the other if the plots are compatible. If the ‘sync’ button is checked, the limits will be synchronized dynamically to any changes made to the other plot, whether made on the Limits Panel or directly in the plot. Multiple plots can be synchronized to a single plot.



When the settings in one tab are copied to another and the Apply button is clicked, other settings, such as the aspect ratio, skew angle, color map, and log settings are also copied. This is therefore a very quick way of making direct comparisons between different data sets.


The plotting pane in the main window cannot be resized this way, because of the constraints of the other panes. Other plotting windows will copy the main window plotting size if requested.

Show Projection Panel

This opens a panel for the currently active plotting window to allow projections along arbitrary axes to be plotted and/or saved. The projections are either two-dimensional or, if the y-box is set to ‘None’, one-dimensional. The projections may be plotted in a separate window, using the ‘Plot’ button or saved to a scratch NXdata group on the tree. If ‘Sum’ is checked, the projection contains the sum over all the summed pixels; if not, the projection contains the average, i.e., the sum divided by the number of pixels in each orthogonal dimension. If a one-dimensional projection is plotted, a checkbox appears allowing additional one-dimensional projections to be plotted over it.

The x and y limits of the plot are displayed as a dashed rectangle, which can be hidden if ‘Hide Limits’ is checked. Dragging with the right-button depressed can be used to change the limits without replotting.


On systems without a right mouse button, right-click dragging can usually be accomplished by other means, e.g., two-finger drags on a trackpad or dragging with the [Ctrl]-key depressed.

All the open projection panels are displayed as tabs in a single window, with the option of copying projection values from one tab to the other if the plots are compatible.



The projection panel can also be used to mask and unmask data within the dashed rectangle. See Python Interface to NeXus for descriptions of masked arrays.

Show Scan Panel

This opens a panel for plotting data across multiple files in the NeXpy tree. The limits are used to define projection of the currently plotted data, which is to be plotted against the variable defined by the path in the Scan field. This path can either be entered manually, or by selecting a scalar quantity in the tree and clicking the ‘Select Scan’ button. The ‘Select Files’ button is then used to define the loaded files to be included in the scan. Values of the scanned variable are automatically read from the file and entered in the box by the corresponding file, where they can be edited if necessary.

Reset Plot Limits

This restores the axis and signal limits to the original values.


This is equivalent to clicking on the Home button in the Options Tab (see below). Right-clicking within the plot restores the axis limits but does not reset the signal limits.

New Plot Window

Opens a new NeXpy plotting window, consisting of a Matplotlib plot pane and its associated axis panels. NeXpy plot commands will be directed to the currently active window. Clicking on the plot pane makes it active. All open windows are listed in the Window menu, along with their labels (‘Main’, ‘Figure 1’, ‘Figure 2’, etc.). These are used to switch the focus for subsequent plots.


If Matplotlib windows are opened from the IPython shell using the standard Pyplot commands, e.g., plt.figure(), they are numbered independently and will not be added to the NeXpy menu. They can be modified using the standard Pyplot commands.

Equalize Plot Sizes

All plot windows are resized to match the main window.

Main, Figure 1, Figure 2…

These menu items set the selected plotting window to be active. As new windows are created, they are dynamically added to this list.

Script Menu

New Script

Opens a new script in an editable text window with syntax coloring. The Python code can be run within the IPython console at any time using the console namespace. That means that all the items on the NeXpy tree are also accessible without further imports.

The scripts can be saved for future use from within NeXpy or from the terminal command line. They can therefore be formatted as a Python standalone script to be either run as python or run in the console (similar to the IPython ‘run magic’, i.e., %run -i Script arguments can be entered in a separate text window at the bottom of the window and accessed within the script in the ‘sys.argv’ list.


Script arguments are just text strings, so if the argument is a node on the tree, it must be referenced as a tree dictionary item, e.g., nxtree[sys.argv[1]]

Scripts are saved, by default, in ~/.nexpy/scripts, and are automatically added to the bottom of the Script Menu.

Open Script

Opens an existing Python script file in an editable text window.


The currently selected node in the NeXpy tree can be referenced in the script as treeview.node.

Open Startup Script

Opens the startup script, ~/.nexpy/, which is run when NeXpy is launched. This can be used to customize imports and other settings that affect the IPython shell.

Help Menus

Open NeXpy Help Online

Opens this documentation in a browser.

Open NeXpy API Tutorial Online

Opens an online Jupyter notebook containing a practical tutorial on the nexusformat package.

Open NeXus Base Classes Definitions Online

Opens online documentation on the current NeXus base classes.

Open IPython Help Online

Opens online documentation on IPython.

Open Intro to IPython

Outputs an introduction to IPython in the shell. Type ‘q’ to return to the shell.

IPython Cheat Sheet

Outputs the IPython Quick Reference Card in the shell. Type ‘q’ to return to the shell.

Open Example File

Launches a ‘Open File’ dialog allowing one of the example NeXus files distributed with NeXpy to be opened.

Open Example Script

Launches a ‘Open File’ dialog allowing one of the example NeXus scripts distributed with NeXpy to be opened in the Script Editor.

Other Menus

The Edit and View Menus consist of menu items provided by the Jupyter Qt Console. All these operations act on the shell text.

Adding NeXus Data to the Tree

NXroot groups that are displayed in the tree pane are all children of a group of class NXtree, known as ‘tree’. If you create a NeXus group dynamically in the IPython shell, it can be added to the tree pane using the tree’s add method:

>>> a=NXroot()
>>> a.entry = NXentry()
>>> nxtree.add(a)

If the group is an NXroot group, it will have the name used in the shell. If the group is not an NXroot group, the data will be wrapped automatically in an NXroot group and given a default name that doesn’t conflict with existing tree nodes, e.g., w4.


The NXroot class is still considered to be the root of the NeXus tree in shell commands. The NXtree group is only used by the GUI and cannot be saved to a file.


In Python, an object may be accessible within the shell with more than one name. NeXpy searches the shell dictionary for an object with the same ID as the added NeXus object and so may choose a different name. The object in the tree can be renamed.

Plotting NeXus Data

NXdata, NXmonitor, and NXlog data can be plotted by selecting a group on the tree and choosing “Plot Data” from the Data menu or by double-clicking the item on the tree (or right-clicking for over-plots). Below the plot pane, a series of tabs allow manipulation of the plot limits and parameters using text boxes and sliders.


The slider ranges are initially set by the data limits. You can redefine the slider ranges by editing their respective minimum and/or maximum text boxes. The original range can be restored by clicking on the Home button in the Options Tab or right-clicking within the plot.

Signal Tab


The signal tab contains text boxes and sliders to adjust the intensity limits, a checkbox to plot the intensity on a log scale, and two dropdown menus to select a color palette and a 2D interpolation method.

The color palettes are divided into three sections, separating perceptually uniform palettes at the top, miscellaneous palettes, and diverging palettes at the bottom. See the Matplotlib documentation for more details.

If a diverging color scale is used, the signal is assumed to be symmetric about 0, so the minimum box and slider are disabled and their values set to the negative of the maximum values. If a log scale is chosen, a symmetric log plot is displayed, with threshold and scale parameters adjustable using the command-line symlog command (see below).


For a one-dimensional plot, there is no signal tab. The intensity is adjusted using the y-tab. There is also no signal tab for an RGB(A) image, since the colors are defined by the RGB(A) values.


The interpolation methods are the default options provided by Matplotlib, which are only available for 2D data with a regular grid.


If the astropy module is installed, the interpolation dropdown menu includes a convolve option. Strictly speaking, this is not an interpolation method, since it performs a Gaussian smoothing of the data, with a standard deviation set by the smooth option (see below). The default is 2 pixels.

X Tab


The x and y-tabs contains text boxes and sliders to adjust the axis limits and a dropdown menu to select the axis to be plotted along x or y, respectively. The names correspond to the axis names in the NXdata group. A checkbox allows the direction of the axes to be flipped.


Flipping the axis directions does not flip the direction of the sliders.

Y Tab


The y-tab has three additions to the features in the x-tab:

  1. Since multiple one-dimensional data sets can be plotted on the same figure, an additional pull-down menu is added on the left-hand side to select them.

  2. Selecting the ‘smooth’ checkbox adds a line that smoothly interpolates one-dimensional data. This uses the SciPy interp1d function. This option is provided to add guides-to-the-eye, and should be used for numerical analysis with caution.

  3. The ‘Fit’ button will open a panel for fitting the data using the LMFIT package.

Z Tab


If the data rank is three or more, the 2D plot vs x and y is a projection along the remaining axes. The z-tab sets the limits for those projections. It contains a dropdown menu for selecting the axis to be averaged or summed over and two text boxes for selecting the projection limits. When the data are first plotted, only the top slice if plotted, i.e., all the z-axis limits are set to their minimum value.


Projections are now averaged over the summed bins by default. To restore the previous behavior, click the ‘Sum’ checkbox in the Projection Tab.

When ‘Lock’ is checked, the difference between the limits of the selected z-axis is fixed. This allows successive images along the z-axis to be plotted by clicking the text-box arrows in increments of the difference between the two limits. If you use the text-box arrows or the terminal arrow keys to change the z-limits when they are locked together, the new plot is updated automatically. Otherwise, the data is only replotted when you force a replot using the toolbar (see below).


Make sure that the value of both limit boxes is entered, e.g., by pressing return after editing their values, before clicking on the ‘lock’ checkbox.

When stepping through the z-values, the ‘Autoscale’ checkbox determines whether the plot automatically scales the signal to the maximum intensity of the slice or is set to the current signal limits.


When ‘Autoscale’ is checked, it is not possible to adjust the limits in the Signal Tab.


The toolbar on the right provides further controls for replotting data as a function of z. The first button on the left forces a replot, e.g., when you have changed z-axis limits or turned on auto-scaling. The other buttons are for stepping through the z-values automatically, with ‘back’, ‘pause’, and ‘forward’ controls. The default speed is one frame per second, but after the first click on the play button, subsequent clicks will reduce the frame interval by a factor two.

Projection Tab


The projection tab allows the data to be projected along one or two dimensions. The limits are set by the x, y, and z-tabs, while the projection axes are selected using the dropdown boxes. For a one-dimensional projection, select ‘None’ from the y box. This is a short-cut to making projections with the Projection Panel.

Options Tab


The options tab is based on the standard Matplotlib toolbar, with the the addition of extra buttons. From left to right, the buttons are:

  • Home - restores all plotting limits to their original values.

  • Arrows - cycles through the limits of previous plots.

  • Pan - enables panning mode (disabling zoom mode).

  • Zoom - enables zoom mode (disabling pan mode).

  • Aspect - toggles between setting the aspect ratio automatically to fill the available space or setting the x and y scales to be equal. This is only valid if the units of the x and y axes are identical.

  • Subplot - configures the spacing around the plot.

  • Edit - opens the Customize Panel to edit both image and point plots. Use this to change the title and axis labels, modify the image aspect ratio and skew angles, turn axis grids on or off and set their styles, modify the point plot markers and lines, scale or add an offset to 1D plots, and draw legends.

  • Save - saves plot to PNG file.

  • Export - exports plotted data to a NeXus file or, for one-dimensional data, a multi-column ASCII file.

  • Add - adds plotted data to the tree pane as an NXdata group within the scratch workspace ‘w0’.

On the far right of the toolbar, the data and axis values are dynamically updated to the values under the current mouse location.

See also

See the Matplotlib documentation for more detailed descriptions of the standard toolbar, including keyboard shortcuts. The ‘Aspect’, ‘Export’, and ‘Add’ buttons are unique to NeXpy.


The aspect ratio of a plot can also be set from the IPython shell. See below.

Command Line Options

It is possible to modify some of the plotting features from the IPython shell. The current plotting pane, the default Matplotlib axis instance, and the current image are exposed as plotview,, and plotview.image, respectively.


Before making any changes, make sure that you have selected the right plotting pane, either by selecting it in the Window menu or using one of the keyboard shortcuts, which are displayed in the menu, e.g., <Ctrl>+2 (⌘+2 on a Mac) to select Figure 2.

  • Set Aspect Ratio:

    >>> plotview.aspect = <aspect>

    <aspect> can be any of the values allowed by the Matplotlib set_aspect function, i.e., ‘auto’, ‘equal’, or the numerical value of the ratio between the height and the width (if the units are identical). The ‘Aspect’ button (see above) toggles between ‘auto’ and ‘equal’. This can also be set using the ‘Edit Parameters’ button on the Options tab.

  • Set Skew Angle:

    >>> plotview.skew = <angle>

    This sets the angle between the x and y-axes in degrees. If set to None, the axes are plotted as orthogonal. If plotview.aspect is currently set to ‘auto’, this command will automatically set it to 1.0 (equivalent to ‘equal’), i.e., assuming the units of the x and y-axes are the same. If they are not, plotview.aspect should be set to the ratio of their units. This can also be set using the ‘Edit Parameters’ button on the Options tab.

  • Set Smoothing Width:

    >>> plotview.smooth = <stddev>

    This sets the standard deviation in pixels for the Gaussian smoothing of the data performed when the ‘convolve’ option is selected in the Signal tab. The default value is 2.

  • Set Offsets:

    >>> plotview.offsets = <True|False>

    If the range of an axis is much smaller than the absolute values, the axis labels can overlap. Setting this option will determine whether Matplotlib converts the axis labels to differences from a fixed offset value or not. The default is False.

  • Select Color Map:

    >>> plotview.cmap = <cmap>

    This allows the color map of the currently displayed image to be changed. This can be useful if the map is not available in the Signal Tab. See the Matplotlib documentation for more details.

  • Draw Shapes:

    >>> plotview.vline(<x>, <ymin>, <ymax>)
    >>> plotview.hline(<y>, <xmin>, <xmax>)
    >>> plotview.vlines(<x-array>, <ymin>, <ymax>)
    >>> plotview.hlines(<y-array>, <xmin>, <xmax>)
    >>> plotview.crosshairs(<x>, <y>)
    >>> plotview.rectangle(<x>, <y>, <dx>, <dy>)
    >>><x>, <y>, <radius>)
    >>> plotview.ellipse(<x>, <y>, <dx>, <dy>)

    These functions draw graphical primitives on the plot using the axis coordinates. In the case of the lines, the complete range of the plot will be used if the minimum and maximum values are omitted. The rectangle coordinates represent the lower left-hand corner but the circle and ellipse coordinates represent the shape center.


    Since the arguments are in the units of the axes, the circle will only be truly circular if the x and y units are the same, and the aspect ratio of the plot is equal.

    All of the functions will accept additional keyword arguments used in drawing Matplotlib shapes, e.g., to change the edge and fill colors, line properties, etc. See the Matplotlib documentation for more details.

  • Draw Grid:

    >>> plotview.grid(True|False)

    Draws grid lines at the major tick values. Additional keyword arguments can be given to modify the color, linestyle, etc, using the standard Matplotlib conventions.

  • Draw Legend:

    >>> plotview.legend(*items, *opts)

    This draws a legend using the standard Matplotlib API, i.e., it is broadly equivalent to calling It is only intended to be used for one-dimensional plots. By default, the labels will contain the full path to each plotted field, but setting the keyword argument, nameonly=True will restrict the label to the field name.


    Legend labels, positions, and other attributes can be modified in the Customize Dialog.

  • Convert to Symmetric Log Plot:

    >>> plotview.symlog(linthresh, linscale, vmax)

    Plot the data using symmetric logarithms for both positive and negative data. The linthresh and linscale parameters are used to define the linear region interpolating between the positive and negative log regions. See the Matplotlib documentation for more details. The maximum and minimum signal values are set to +/- vmax.

    Calling symlog will set the linthresh and linscale parameters for future plots. Call it without any parameters to set them to their default values, linthresh=vmax/10 and linscale=0.1.


    There are a number of diverging color maps, such as coolwarm, that are ideal for displaying symmetric log data. Some are available at the bottom of the color map dropdown menu in the Signal tab.

Keyboard Shortcuts

A number of keyboard shortcuts are defined when focus is on the plotting window. These can be used to switch between tabs or set various plotting options.


Keyboard focus can be switched to a particular plotting window by (a) clicking within the window, (b) using the Window menu, or (c) typing Ctrl+’n’ (⌘+’n’ on a Mac), where ‘n’ is the plot window number.

  • s - switch to the Signal tab.

  • x - switch to the X tab.

  • y - switch to the Y tab.

  • z - switch to the Z tab.

  • p - switch to the Projection tab.

  • o - switch to the Options tab.

  • l - toggle logarithmic signal scale (2D plots only).

  • g - toggle display of major and minor grid.

  • G - toggle display of major grid.

  • P - toggle panning mode (if enabled, zoom mode is disabled).

  • Z - toggle zoom mode (if enabled, pan mode is disabled).

  • E - toggle the aspect ratio between ‘equal’ and ‘automatic’.

  • S - save plot to a graphics file.

  • A - add plotted data to the tree pane.

  • O - open dialog to customize plots.

Configuring NeXpy

When NeXpy if first launched, a private directory is created in the home directory, ~/.nexpy/. This is used to store log files, backups, plugins, and scripts. A configuration file, ~/.nexpy/, is created to contain Python commands that should be run at the start of every session.

By default, the configuration file contains a number of imports, including all the functions and classes defined by the nexusformat package.

import nexpy
import as nx
from import *

This file could also be used to change the default parameters used by the nexusformat package to define, e.g., memory limits, maximum loaded array sizes, file locking, default HDF5 compression, and default string encodings. See Python Interface to NeXus for more details.

For convenience, the configuration file also imports a number of other modules that are commonly used:

import sys
import os
import h5py as h5
import numpy as np
import as ma
import scipy as sp
import matplotlib as mpl
from matplotlib import pylab, mlab, pyplot
plt = pyplot

If you require a different set of imports or prefer alternative abbreviations, edit the configuration file using``Open Startup Script…`` in the Script Menu.

Fitting NeXus Data

NeXpy makes it easy to fit one-dimensional data using the LMFIT package, with a ‘Fit’ button in the Y-Tab of every one-dimensional plot.


If multiple data sets are plotted in the same window, the one to be fit can be selected using the pull-down menu on the far left of the Y-Tab. Multiple data sets can be selected for fitting at the same time, each one opening a new tab in the Fit Panel. Line plots of the models and their components will be plotted in the same color as the corresponding data.

Alternatively, choosing ‘Fit Data’ from the Data menu or using the keyboard shortcut Ctrl+Shift+F (⌘+⇧+F on a Mac), will fit data selected in the Tree Pane.

Either method opens a dialog window that allows multiple fit models to be combined, with the option of fixing or limiting parameters. To help in selecting a model, click on the pull-down menu and the model description will be displayed as a tooltip when you hover over it.


The fit can be plotted, along with the constituent models in the main plotting window and the fitting parameters displayed in the Fit Panel.


The data are only fitted within the x-limits of the current plot. This can be used, for example, to perform piece-wise fits of multiple peaks before a final fit that combines them all together.

Initializing Parameters

LMFIT models often define a function to guess initial parameters from the data. If multiple peaks are to be fitted, sensible starting parameters for each one can be determined by moving the x-limits to cover a single peak when adding the peak model. Then, the x-limits can be restored before fitting all the peaks together.

Masking Data

Data points can be masked so that they are excluded from the fit. Individual points can be removed by double-clicking on the point marker. A set of x-values can be excluded by right-click dragging over the required range and then clicking on ‘Mask Data’. Masks can be cleared by clicking on ‘Clear Masks’.

Modifying Constraints

Parameters can be fixed or constrained with minimum and maximum limits in the Fit Panel. However, LMFIT also allows parameters to be bound to the values of one or more other parameters by algebraic expressions. These expressions can be defined or modified by clicking on ‘Σ’ button at the end of the parameter row. Pull-down menus allow parameters from any of the currently added models to be inserted into these expressions.

Composing Models

When models are added using the ‘Add Model’ button, they are combined to produce a composite model, in which they are added together by default. However, LMFIT allows composite models to be combined using different operators (add, subtract, multiply, and divide), defined by an algebraic expression. For example, a BoseFactor model, with temperature its only parameter, is provided by NeXpy. If it is combined with a peak model, using the ‘multiply’ operator, it will apply a detailed balance factor appropriate for modeling quasielastic neutron scattering.

A ‘Compose Model’ button allows the algebraic expression combining the currently added models to be edited.

It is also possible to combine a subset of models when plotting the fitted models, by selecting ‘Composite Model’ before clicking ‘Plot Model’. This allows, for example, several functions representing a background to be combined before they are plotted.

Saving the Fit

The original data, the fitted data, constituent models, and the parameters can all be saved to an NXprocess group in the Tree Pane, using the ‘Save Fit’ button, for subsequent plotting, refitting, or copying to another NeXus file. The group, named ‘f1’, ‘f2’, etc., is stored in the default scratch NXroot group, w0. If you choose to fit this entry again, it will load the models and parameters from the saved fit.

Closing the Fit Panel

If the Fit Panel was opened by clicking the ‘Fit’ button in Y-Tab, line plots of the models and their components are superposed on the existing plot window. These line plots will be erased when the corresponding tab in the Fit Panel is closed. However, if the ‘Apply’ button is clicked before closing the tab, the line plot representing the combined model will be preserved until the plot window is closed.

Defining a Model

NeXpy makes available any of the models currently supplied by the LMFIT package, as well as a couple of extra models added to the NeXpy package, the OrderParameterModel and the PDFdecayModel. If you wish to construct your own model, please refer to the LMFIT documentation for more details.

User-defined models can be added as separate files to their private models directory in ~/.nexpy/models (new to v0.12.6). As an example, here is the code for the OrderParameterModel that is distributed with NeXpy:

import numpy as np

from lmfit.model import Model

class OrderParameterModel(Model):
    r"""A model to describe the temperature dependence of an order parameter
    with three Parameters: ``amplitude``, ``Tc``, and ``beta``.

    .. math::

        f(x; A, Tc, \beta) = A ((Tc - x[x<Tc])/ Tc)^\beta

    where the parameter ``amplitude`` corresponds to :math:`A`, ``Tc`` to
    :math:`Tc`, and ``beta`` to :math:`\beta`.
    def __init__(self, **kwargs):

        def op(x, amplitude=1.0, Tc=100.0, beta=0.5):
            v = np.zeros(x.shape)
            v[x<Tc] = amplitude * ((Tc - x[x<Tc])/ Tc)**beta
            v[x>=Tc] = 0.0
            return v

        super().__init__(op, **kwargs)

    def guess(self, data, x=None, negative=False, **kwargs):
        """Estimate initial model parameter values from data."""
        return self.make_params(amplitude=data.max(), Tc=x.mean(), beta=0.33)


Prior to v0.12.6, NeXpy defined its own system for generating fitting functions. This system is now deprecated, but legacy functions are still available at the end of the model list. If you have produced your own functions in the past, they will also be on this list. However, we recommend that all new functions now adhere to LMFIT model definitions.

Importing NeXus Data

NeXpy can import data stored in a number of other formats, including SPEC files, TIFF images, and text files, using the File:Import menus. If a file format is not currently supported, the user can write their own. The following is an example of a module that reads the original format and returns NeXus data:

def get_data(filename):
    from libtiff import TIFF
    im =
    z = im.read_image()
    y = range(z.shape[0])
    x = range(z.shape[1])
    return NXentry(NXdata(z,(y,x)))

This could be run in the shell pane and then added to the tree using:

>>> nxtree.add(get_data('image.tif'))

Existing Readers

NeXpy is currently distributed with readers for the following format:

SPEC Files

This reader will read multiple SPEC scans from a single SPEC log file, creating a separate NXentry for each scan. All the columns in each scan are read into the NXdata group, with the default signal defined by the last column. Mesh scans are converted to multi-dimensional data, with axes defined by the scan command. It is possible to plot different columns once the scans are imported.

TIFF Images

This reader will import most TIFF images, including those with floating point pixels. This currently uses the tifffile module. Use the Open Image... dialog to use the FabIO library.

Image Stack

This reader will read a stack of images, which are readable by FabIO, e.g., TIFF or CBF, into a three-dimensional NXdata group. The image stack must be stored in separate files in a single directory, that are grouped with a common prefix followed by an integer defining the stack sequence.

Text Files

This reader will read ASCII data stored in two or three columns, containing the x and y values, and, optionally, errors. One or more header lines can be skipped. A more flexible text importer, allowing the selection of data from multiple columns, is under development.

Defining a Reader

It is possible to add a reader to the File:Import menu using the existing samples as a guide in the nexpy.readers directory. User-defined import dialogs can be added to their private readers directory in ~/.nexpy/readers.

Here is an example of an import dialog:

Module to read in a TIFF file and convert it to NeXus.

Each importer needs to layout the GUI buttons necessary for defining the
imported file and its attributes and a single module, get_data, which returns
an NXroot or NXentry object. This will be added to the NeXpy tree.

Two GUI elements are provided for convenience:

    ImportDialog.filebox: Contains a "Choose File" button and a text box. Both
                          can be used to set the path to the imported file.
                          This can be retrieved as a string using
    ImportDialog.close_buttons: Contains a "Cancel" and "OK" button to close
                                the dialog. This should be placed at the bottom
                                of all import dialogs.

import numpy as np
from import *
from nexpy.gui.importdialog import NXImportDialog

filetype = "TIFF Image" #Defines the Import Menu label

class ImportDialog(NXImportDialog):
    """Dialog to import a TIFF image"""

    def __init__(self, parent=None):


        self.set_layout(self.filebox(), self.close_buttons())

        self.set_title("Import "+str(filetype))

    def get_data(self):
        from libtiff import TIFF
        im =
        z = NXfield(im.read_image(), name='z')
        y = NXfield(range(z.shape[0]), name='y')
        x = NXfield(range(z.shape[1]), name='x')
        return NXentry(NXdata(z,(y,x)))

NeXpy Plugins

It is possible to customize NeXpy by adding new menus to the main menu bar with sub-menus that open GUI dialogs or perform operations that are specific to a specialized application.

Installing Plugins

NeXpy searches for plugin modules in two ways.

  1. The plugin code can be installed using the Install Plugin... dialog either locally in the user’s ~/.nexpy/plugins``directory or in the ``nexpy.plugins directory within the installed NeXpy distribution.

  2. The plugin code can be contained within an external installed package, which declares an entry point labelled nexpy.plugins. An example package, that is installable using pip install . is available in the NeXpy package examples directory.


    The second method was introduced in NeXpy v1.0.3. It is recommended for new plugins.

Plugins are loaded from the users’ directory, the NeXpy distribution’s plugin directory, and external package entry points, in that order, for backward compatibility with existing installations. Duplicate plugins will not be loaded, but a warning will be added to the NeXpy log file. If a previously installed plugin is now available in an external package, please remove the prior installation using the Remove Plugin... dialog.

Defining Plugins

The modules that are accessed through the plugin menu should be defined as a Python package, i.e., by creating a sub-directory that contains This must contain a function, plugin_menu, which returns the name to be added to the top-level NeXpy menu bar, and a list of tuples, each of which contains the sub-menu name and the corresponding function triggered by clicking on it.

There is an example package, chopper_plugin, in the nexpy.examples directory, to show how plugins should be configured. It adds a top-level menu item, Chopper, that has a couple of sub-menu items to perform data analysis on the example file, chopper.nxs, which is also distributed with NeXpy.

Here is the structure of the chopper_plugin package:

└── pyproject.toml
└── chopper
    ├── __init.py__


If the plugin is to be installed using the Install Plugin... dialog, just select the chopper sub-directory in the above file tree.

Here is the pyproject.toml file:

requires = ["setuptools", "setuptools-scm"]
build-backend = "setuptools.build_meta"

name = "chopper_plugin"
version = "1.0.0"
dependencies = [

chopper = "chopper:plugin_menu"

This is sufficient to install the plugin using pip and make it discoverable by NeXpy through nexpy.plugins entry point. If the plugin is embedded within a larger package, adjust the entry point so that it points to the sub-directory containing the file that defines the plugin_menu function. For example, if the plugin modules are contained within the plugins sub-directory of mypackage, add the following entry point:

plugin_name = "mypackage.plugins.plugin_name:plugin_menu"

See also

Information on defining entry points in external packages that do not use a pyproject.toml file is available in the Setup Tools documentation.

Here is the file, which defines the plugin_menu function:

from . import get_ei, convert_qe

def plugin_menu():
    menu = 'Chopper'
    actions = []
    actions.append(('Get Incident Energy', get_ei.show_dialog))
    actions.append(('Convert to Q-E', convert_qe.show_dialog))
    return menu, actions

The actions define the menu text and the function that gets called when it is selected. In the example, they are contained within the package as two files, and, but they could also be in a separately installed package in the Python path.

These files should open a dialog box and perform the required operations, after which the results can either be saved to a new NeXus file or saved as modifications to an existing tree item.

For example, reads the monitor spectra contained within the currently selected node on the tree, which should have been previously loaded. It then calculates the difference between the peak positions of the two spectra, calculates the incident energy, which is updated in both the dialog box and, if the Save button is pressed, in the loaded NeXus tree, ready for subsequent analysis.

In the simplest cases, no knowledge of PyQt is required. In the example below, a grid defines a set of parameters, functions to read those parameters from the PySide text boxes (here, they are decorated with @property, which means that the function can be called without an argument), a couple of buttons to activate different parts of the analysis, and finally the functions themselves:

import numpy as np
from nexpy.gui.datadialogs import GridParameters, NXDialog
from nexpy.gui.mainwindow import report_error
from import NeXusError

def show_dialog(parent=None):
        dialog = EnergyDialog()
    except NeXusError as error:
        report_error("Getting Incident Energy", error)

class EnergyDialog(NXDialog):

    def __init__(self, parent=None):

        super(EnergyDialog, self).__init__(parent)

        self.parameters = GridParameters()
        self.parameters.add('m1', self.entry['monitor1/distance'],
                            'Monitor 1 Distance')
        self.parameters.add('m2', self.entry['monitor2/distance'],
                            'Monitor 2 Distance')
                            'Incident Energy')
        self.parameters.add('mod', self.entry['instrument/source/distance'],
                            'Moderator Distance')
        action_buttons = self.action_buttons(('Get Ei', self.get_ei))
        self.set_layout(self.entry_layout, self.parameters.grid(),
                        action_buttons, self.close_buttons(save=True))
        self.set_title('Get Incident Energy')

        self.m1 = self.entry['monitor1']
        self.m2 = self.entry['monitor2']

    def m1_distance(self):
        return self.parameters['m1'].value - self.moderator_distance

    def m2_distance(self):
        return self.parameters['m2'].value - self.moderator_distance

    def Ei(self):
        return self.parameters['Ei'].value

    def moderator_distance(self):
        return self.parameters['mod'].value

    def get_ei(self):
        t = 2286.26 * self.m1_distance / np.sqrt(self.Ei)
        m1_time = self.m1[t-200.0:t+200.0].moment()
        t = 2286.26 * self.m2_distance / np.sqrt(self.Ei)
        m2_time = self.m2[t-200.0:t+200.0].moment()
        self.parameters['Ei'].value = (2286.26 *
                                       (self.m2_distance - self.m1_distance) /
                                       (m2_time - m1_time))**2

    def accept(self):
        except NeXusError as error:
            report_error("Getting Incident Energy", error)
        super(EnergyDialog, self).accept()