title: several contour plots in the same figures
body: <p>I have several 3d functions. I would like two plot the contour plots of them in the same figure to see the difference between them. I expect to see some crossings between contours of two functions. Here is my code:</p>
  
  <pre><code>plt.contour(xi, yi, F)
  plt.contour(xi, yi, F1)        
  plt.show()
  </code></pre>
  
  <p>But, it seems that the first one is erased at the end, since I see only one function without any crossing of contours. Is it possible to figure this out somehow?</p>