title: Outline a region in a graph
body: <p>I have two 2D numpy arrays (of the same dimensions) that I am plotting using matplotlib.  The first array I've plotted as a color map in gray-scale.  The second one represents an aperture, but it is an irregular shape (some of the pixels get outlined, and it is a set of horizontal and vertical lines that form the outline).  I am not sure how to ask it to plot this second array.  The array is composed of three numbers (0, 1, and 3), and I only need the pixels of one value (3) to be outlined, but I need the outline to encompass the region of these pixels, not the pixels individually.  I need the interior of all the pixels to remain transparent so that I can see the gray-scale color map through it.</p>
  
  <p>Does anyone know how to accomplish this?  </p>