title: Making a logarithmic scaled color plot including negative values
body: <p>I have a surface density map in matplotlib for which I want the scale of the colorbar to be logarithmic.</p>
  
  <p>Ideally using the <code>LogNorm</code> function should do the trick, but there are negative and null values, which gives an error when the values are turned logarithmic.<br>
  Is there function/method that I can use to include the areas with negative and null values in this respect?</p>