title: Overlay polygon on top of image in Python
body: <p>Say I have an image in PIL</p>
  
  <pre><code>from PIL import Image
  Image.open(path_to_my_image)
  </code></pre>
  
  <p>and two lists of <code>x</code> points and <code>y</code> points</p>
  
  <pre><code>x = ['10', '30', '70']
  y = ['15', '45', '90']
  </code></pre>
  
  <p>Is there a way to overlay my polygon <strong>with transparency</strong> on this image?</p>
  
  <p>Also, is PIL a good library for this? Or should I use a different one? (e.g. <code>scikits.image</code>, or render in an image with pylab).</p>