Font not found for matplotlib

[tahorst]

After #1265, there is sometimes an issue finding a font on the system and you'll see the following message:

findfont: Font family ['sans-serif'] not found. Falling back to DejaVu Sans.

I found you can get around this by removing a matplotlib cache so that matplotlib rebuilds the font cache the next time it runs:

rm -r ~/.cache/matplotlib/

Apparently, Sherlock only has dejavu sans installed as a font so you might need to copy arial.ttf from $PI_HOME/.fonts/arial.ttf (this is from my local font directory /usr/share/fonts/truetype/msttcorefonts/arial.ttf) to ~/.fonts/ and then remove the matplotlib cache.