title: matplotlib show() method does not open window
body: <p>I'm using a mac and when I do the following with matplotlib:</p>
  
  <pre><code>import matplotlib.pyplot as plt
  import matplotlib.mlab as mlab
  import pylab as P
  
  ...
  plt.plot(x,y)  
  plt.show() &lt;-- nothing happens
  plt.savefig('figure.png') &lt;-- works fine
  </code></pre>
  
  <p>So, <code>plt.show</code> does not open a window or anything while <code>plt.savefig</code> works fine.</p>
  
  <p>What could be the problem?</p>