title: Contour from 2D image
body: <p>I'm wondering if it's possible to plot a contour (level curves) graphic from a 2D image in Python. More specific, I'm wondering how could I read an image data (the set of the grayscale pixels, for example), and use it for the z input
  I'm reading some articles about the matplotlib but I couldn't find an example which the input is an image.</p>