title: In matplotlib imshow() how can I change the color axis range on an existing plot?
body: <p>When making a plot to start with I can use:</p>
  <pre><code>fig, ax = plt.subplots() 
  h = ax.imshow(X, vmin=mn, vmax=mx)
  </code></pre>
  <p>Once the plot is up, can I change my mind and make <code>vmin</code> and <code>vmax</code> something different?</p>