title: Matplotlib change colormap tab20 to have three colors
body: <p>Matplotlib has some new and very handy colormaps (<a href="https://matplotlib.org/examples/color/colormaps_reference.html" rel="noreferrer">tab colormap</a>). What I miss is a feature to generate a colormap like tab20b or tab20c but with three hue levels instead of four ? </p>
  
  <p>This solution is a bit complicated, is there an easier one ?</p>
  
  <pre><code>skip = []
  for i in range(0,len(cm.colors)//4+1):
      skip.append(4*i)
  # the colormap is called Vega in my Matplotlib version
  cm = plt.cm.get_cmap('Vega20c')
  cm_skip = [cm.colors[i] for i in range(len(cm.colors)) if i not in skip]
  
  for i, c in enumerate(cm_skip):
      x = np.linspace(0,1)
      y = (i+1)*x + i
      plt.plot(x, y, color=c, linewidth=4)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/0A8G5.png" rel="noreferrer"><img src="https://i.stack.imgur.com/0A8G5.png" alt="enter image description here"></a></p>
  
  <p>The colormap in Matplotlib:
  <a href="https://i.stack.imgur.com/uaJI7.png" rel="noreferrer"><img src="https://i.stack.imgur.com/uaJI7.png" alt="enter image description here"></a></p>
  
  <p>EDIT: A more generic approach has been provided in this <a href="https://stackoverflow.com/questions/47222585/matplotlib-generic-colormap-from-tab10">SO post</a>.</p>