title: Change the labels of a colorbar from increasing to decreasing values
body: <p>I'd like to change the labels for the colorbar from increasing to decreasing values. When I try to do this via <code>vmin</code> and <code>vmax</code> I get the error message:</p>
  
  <blockquote>
    <p>minvalue must be less than or equal to maxvalue</p>
  </blockquote>
  
  <p>So, for example I'd like the colorbar to start at 20 on the left and go up to 15 on the right.</p>
  
  <p>This is my code for the colorbar so far, but in this example the values go from 15 to 20 and I'd like to reverse that order:</p>
  
  <pre><code>cmap1 = mpl.cm.YlOrBr_r
  
  norm1 = mpl.colors.Normalize(15,20)
  
  cb1   = mpl.colorbar.ColorbarBase(colorbar1, cmap=cmap1, norm=norm1, orientation='horizontal')
  
  cb1.set_label('magnitude')
  </code></pre>