Matplotlib is awesome. I just wish the API was cleaner:

  figure.set_frameon()
  axis.set_frame_on()

  pyplot.xlim()
  axis.set_xlim()

This is really pain to deal with when you convert a plot into a subplot and have to switch to the different axis API. Some of this is damage from copying Matlab's legacy but it could use a polish. If you don't use an editor with IntelliSense you have to constantly look up methods in the docs to find the right spelling.