Once you have homebrew, python, and numpy installed as indicated in the post, above, do as follows in a terminal:
brew install freetype
brew install libpng
chmod +w /usr/local/lib
brew link freetype --force --overwrite
The last line should fail, but list a couple of files. Find the files (it tells you where they are) and delete them (or move them into a new folder for safe keeping). Retype the command and the link should occur. Then:
chmod +w /usr/local/include
brew link libpng --force --overwrite
As in the previous case, this will fail and list a few files. Proceed as in the previous case. Retype the command.
pip install matplotlib
pip install git+git://github.com/matplotlib/matplotlib.git#egg=matplotlib-dev
You'll have to use the latter if you're using the Mountain Lion OS. It was the latter that I concluded with, personally.