Error when starting plotting tool: "Cannot load backend 'Qt5Agg' which requires the 'qt5' interactive framework, as 'headless' is currently running"

[bobeldering]

The error message:

Error running pySCHED: Cannot load backend 'Qt5Agg' which requires the 'qt5' interactive framework, as 'headless' is currently running

Background:

pySCHED uses PyQt5 to create the plotting GUI. Since version 5.12 of this library, it has external dependencies that are not automatically installed. To get to the details of this error, you can create a QApplication object in Python with the environment variable QT_DEBUG_PLUGINS set to 1. For example like this:

$ env QT_DEBUG_PLUGINS=1 python
Python 3.9.7 (default, Sep 16 2021, 13:09:58) 
[GCC 7.5.0] :: Anaconda, Inc. on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from PyQt5.QtWidgets import QApplication; QApplication([])

This will print out all the plugins that Qt is trying to load. The last few lines should look similar to this:

Got keys from plugin meta data ("xcb")
QFactoryLoader::QFactoryLoader() checking directory path "/home/eldering/anaconda3/envs/pySCHED/bin/platforms" ...
Cannot load library /home/eldering/anaconda3/envs/pySCHED/lib/python3.9/site-packages/PyQt5/Qt/plugins/platforms/libqxcb.so: (libxkbcommon-x11.so.0: cannot open shared object file: No such file or directory)
QLibraryPrivate::loadPlugin failed on "/home/eldering/anaconda3/envs/pySCHED/lib/python3.9/site-packages/PyQt5/Qt/plugins/platforms/libqxcb.so" : "Cannot load library /home/eldering/anaconda3/envs/pySCHED/lib/python3.9/site-packages/PyQt5/Qt/plugins/platforms/libqxcb.so: (libxkbcommon-x11.so.0: cannot open shared object file: No such file or directory)"
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx, webgl, xcb.

Aborted

One line is of particular interest:

Cannot load library /home/eldering/anaconda3/envs/pySCHED/lib/python3.9/site-packages/PyQt5/Qt/plugins/platforms/libqxcb.so: (libxkbcommon-x11.so.0: cannot open shared object file: No such file or directory)
Indicating that the library libxkbcommon-x11.so.0 is missing.

Resolution

This missing library can be installed with:

sudo apt install libxkbcommon-x11-0