From the website:
matplotlib is a python 2D plotting library which produces publication quality figures in a variety of hardcopy formats and interactive environments across platforms. matplotlib can be used in python scripts, the python and ipython shell (ala MATLAB®* or Mathematica®†), web application servers, and six graphical user interface toolkits.
If the name sounds familiar, it should as it is mentioned in scikits.learn machine learning in Python, but I didn’t have a link to the package.