title: matplotlib plot and imshow
body: <p>The behavior of matplotlib's plot and imshow is confusing to me.</p>
  <pre><code>import matplotlib as mpl
  import matplotlib.pyplot as plt
  </code></pre>
  <p>If I call <code>plt.show()</code> prior to calling <code>plt.imshow(i)</code>, then an error results. If I call <code>plt.imshow(i)</code> prior to calling <code>plt.show()</code>, then everything works perfectly. However, if I close the first figure that gets opened, and then call <code>plt.imshow(i)</code>, a new figure is displayed without ever calling <code>plt.show()</code>.</p>
  <p>Can someone explain this?</p>