cannot change font to Helvetica in Matplotlib in Python on Mac OS X 10.6

The solution is to use fondu to convert the .dfont Helvetica font from Mac OS X into .ttf, and then place that in the mpl-data/fonts directory that Matplotlib looks in. That solved the issue.

Step-by-step solution for OS X 10.11 El Capitan and Python 3 (based on this post).

1. Install fondu: brew install fondu

2. Find out matplotlib location:

   python3 -c "import matplotlib ; print(matplotlib.matplotlib_fname())" 
   

   For me it's /usr/local/lib/python3.5/site-packages/matplotlib/mpl-data/matplotlibrc

3. Make a copy of Helvetica:

   mkdir ~/Desktop/font_copies
   cp /System/Library/Fonts/Helvetica.dfont ~/Desktop/font_copies
   

4. Convert the Helvetica copy we've made from dfont to ttf:

   cd /usr/local/lib/python3.5/site-packages/matplotlib/mpl-data/fonts/ttf/
   fondu -show ~/Desktop/font_copies/Helvetica.dfont
   

5. Remove font cache: rm ~/.matplotlib/fontList.py3k.cache

Done! Now you can use Helvetica:

import matplotlib.pyplot as plt
plt.rc('font', family='Helvetica')