Matplotlib cannot find basic fonts

To get it to work, I had to combine the two current top answers. Here's what worked for me:

$ sudo apt install msttcorefonts -qq
$ rm ~/.cache/matplotlib -rf

I had this exact same problem on a Vagrant VM running Ubuntu Xenial 64-bit. No matter how many fonts I had already installed, matplotlib was having a problem with the "system" font name "sans-serif". I fixed it by:

• Stopping Jupyter
• Installing font-manager: sudo apt install font-manager
• Cleaning the matplotlib cache directory: rm ~/.cache/matplotlib -fr
• Restarting Jupyter.

No more error messages about sans-serif.

This work for me::

$ sudo apt-get install msttcorefonts -qq

A solution for Windows users, when confronted with the warning:

UserWarning: findfont: Font family ['serif'] not found. Falling back to DejaVu Sans
(prop.get_family(), self.defaultFamily[fontext]))

1. Delete the fonts located in matplotlib's cache.
   Cache's location: import matplotlib as mpl; print(mpl.font_manager.get_cachedir())

2. Find matplotlib's font directory. The path might be similar to
   C:\Miniconda3\pkgs\matplotlib-2.2.2-py36_1\Lib\site-packages\matplotlib\mpl-data\fonts\ttf

3. Copy necessary fonts like Computer Modern to this directory.

The warning may persist, but your plots' font should change appropriately.

I actually solved by

import matplotlib.font_manager