Kwant plotter uses deprecated matplotlib functionality
This error is seen when building the docs:
208 /builds/kwant/kwant/build/lib.linux-x86_64-3.6/kwant/_plotter.py:177: MatplotlibDeprecationWarning: 209 The makeMappingArray function was deprecated in Matplotlib 3.2 and will be removed two minor releases later. This is not considered public API any longer.
This means that the plotter may start breaking with bleeding-edge matplotlib in the future.