title: Zero-value colour in matplotlib hexbin
body: <p>I have some spatially-distributed data. I'm plotting this with <code>matplotlib.pyplot.hexbin</code> and would like to change the "background" (i.e. zero-value) colour. An example is shown below - my colour-map of choice is <code>matplotlib.cm.jet</code>:</p>
  
  <p><img src="https://i.stack.imgur.com/X1UzW.png" alt="Example data"></p>
  
  <p>How can I change the base colour from blue to white? I have done something similar with masked arrays when using <code>pcolormesh</code>, but I can't see anyway of doing so in the <code>hexbin</code> arguments. My instinct would be to edit the colourmap itself, but I've not had much experience with that.</p>
  
  <p>I'm using matplotlib v.0.99.1.1</p>