title: How to draw a rectangle over a specific region in a matplotlib graph
body: <p>I have a graph, computed from some data, drawn in matplotlib.  I want to draw a rectangular region around the global maximum of this graph.  I tried <code>plt.axhspan,</code> but the rectangle doesn't seem to appear when I call <code>plt.show()</code></p>
  
  <p>So, how can a rectangular region be drawn onto a matplotlib graph?  Thanks!</p>