title: how to annotate heatmap with text in matplotlib
body: <p>I am plotting a heatmap in matplotlib using:</p>
  
  <p><code>plt.pcolor(rand(5,5))</code></p>
  
  <p>how can I annotate the heatmap with the actual numbers plotted? meaning in each cell of the plotted heatmap, put the value corresponding to that cell in the 5x5 matrix passed to <code>pcolor</code>. thanks.</p>