If packages is installed, this should never happen. I should search for python-matplotlib-qt4 in dependency instead of python-matplotlib-qt. If would be good, it gui can also use PyQt5. That would ...

Related to #1065 (but opening a new issue as this is not a Qt problem but how the matplotlib rc is patched AFAIU). Also reported it at matplotlib: matplotlib/matplotlib#7124 (comment) When you do a ...