title: matplotlib contourf: get Z value under cursor
body: <p>When I plot something with <code>contourf</code>, I see at the bottom of the plot window the current x and y values under the mouse cursor.
  Is there a way to see also the z value?</p>
  
  <p>Here an example <code>contourf</code>:</p>
  
  <pre><code>import matplotlib.pyplot as plt
  import numpy as hp
  plt.contourf(np.arange(16).reshape(-1,4))
  </code></pre>