title: Set color to be used for an array masked values plotted with matplotlib contourf
body: <p>When I plot an array with masked values, using matplotlib <code>countourf</code>, the masked values appear white. I want them to appear grey.</p>
  
  <p>I tried the <code>set_bad</code> method, but it seems <code>countourf</code> doesn't recognize it (although it recognizes the <code>set_over</code> and <code>set_under</code> methods).</p>
  
  <p>Is there any other method I can use with <code>contourf</code>?</p>
  
  <p>Or will I have to change my code to use <code>imshow</code>, which understands <code>set_bad</code>, instead of <code>countourf</code>?</p>