While the question has been answered, I'd like to add some useful tips when using matplotlib.pyplot.savefig. The file format can be specified by the extension:

from matplotlib import pyplot as plt

plt.savefig('foo.png')
plt.savefig('foo.pdf')

Will give a rasterized or vectorized output respectively, both which could be useful. In addition, you'll find that pylab leaves a generous, often undesirable, whitespace around the image. Remove it with:

savefig('foo.png', bbox_inches='tight')