title: Map value to specific color in seaborn heatmap
body: <p>I am plotting a heatmap in Python with the seaborn package. The values I am plotting are discrete, they are the integers <code>-1</code>, <code>0</code>, and <code>1</code>. </p>
  
  <p>I would like the cells in the heatmap with the value <code>-1</code> to show up green, those with <code>0</code> as yellow, and <code>1</code> as red. </p>
  
  <p>Is it possible to specify this ruling in the <code>cubehelix_palette()</code> or <code>colour_palette()</code> functions?</p>