matplotlib2 fails to build with Sphinx 1.8, currently available inexperimental:  # build the doc  [...] /usr/bin/make -C doc html  make[1]: Entering directory '/<<PKGBUILDDIR>>/doc'  Running Sphinx v1.8.3  Warning, treated as error:  while setting up extension matplotlib.sphinxext.mathmpl: directive u'math' is already registered, it will be overridden  make[1]: *** [Makefile:20: html] Error 2This was fixed in matplotlib 3.x in this commit, please backport it:https://github.com/matplotlib/matplotlib/commit/f87e9f46daf42ad7That commit switches from generating PNG images for math formulas to MathJaxlibrary, which renders the math formulas on client side. To package it forDebian, please add this line to conf.py:  mathjax_path = 'file:///usr/share/javascript/mathjax/MathJax.js?config=TeX-AMS-MML_HTMLorMML'and make your documentation package depend on libjs-mathjax.Also upstream has noticed that some math renders incorrectly with MathJax,there is a follow-up commit which fixes it:https://github.com/matplotlib/matplotlib/commit/a101de79650e077a