title: pyplot colormap with extend option (in contourf)
body: <p>I have a set of data ranging from -0.5 to 2 (for example). I only want to color the data that are within [-0.5,1]. I did something like:</p>
  
  <pre><code>from matplotlib import colors as mpl_colors
  
  color_map  =  plt.cm.jet
  color_map.set_bad('gray',  1.0)
  norm_color = mpl_colors.Normalize(vmin=-0.5, vmax=1, clip=False)
  nb_colors = 40
  map_frame.contourf(x_mesh, y_mesh, data, nb_colors, cmap=color_map, norm=norm_color, extend='both')
  </code></pre>
  
  <p>I expect the colorbars to have 40 colored strides, ranging from -0.5 to 1.</p>
  
  <p>Instead I get the following image:</p>
  
  <p><a href="https://i.stack.imgur.com/b4rU4.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/b4rU4.png" alt="Results with weird colorbar"></a></p>
  
  <p>The colorbar doesnot stop at 1 as expected, like it does for example here: <a href="http://matplotlib.org/examples/pylab_examples/contourf_demo.html" rel="nofollow noreferrer">http://matplotlib.org/examples/pylab_examples/contourf_demo.html</a></p>
  
  <p>Do you know why ?</p>