title: Create own colormap using matplotlib and plot color scale
body: <p>I have the following problem, I want to create my own colormap (red-mix-violet-mix-blue) that maps to values between -2 and +2 and want to use it to color points in my plot.
  The plot should then have the colorscale to the right.<br>
  <br>
  That is how I create the map so far. But I am not really sure if it mixes the colors.</p>
  
  <pre><code>cmap = matplotlib.colors.ListedColormap(["red","violet","blue"], name='from_list', N=None)
  m = cm.ScalarMappable(norm=norm, cmap=cmap)
  </code></pre>
  
  <p><br>That way I map the colors to the values.<br></p>
  
  <pre><code>colors = itertools.cycle([m.to_rgba(1.22), ..])
  </code></pre>
  
  <p><br>Then I plot it:<br></p>
  
  <pre><code>for i in range(0, len(array_dg)):
    plt.plot(array_dg[i], markers.next(),alpha=alpha[i], c=colors.next())
  </code></pre>
  
  <p><br>
  My problems are:<br>
  1. I can't plot the color scale.<br>
  2. I am not completely sure if my scale is creating a continues (smooth) colorscale.</p>