title: how to make colors in line graph fade in matplotlib and python
body: <p>I have two figures with 3 subplots in each of them.  In each subplot, there are 20 different curves, which represents 20 steps, by using a for loop.  How can I make the color of the curves gradually fade?  Like in the code I have below, the top subplot (311) has 20 blue curves...how can I make the 1st step be dark blue and have the blue gradually fade until the last step be a light blue?  Also, how do I make the two figures pop up on screen at once?  Right now, I have to manually close the first figure in order for the second figure to pop up.</p>
  
  <pre><code>from pylab import *
  for raw_step in raw:
      raw_step = zip(*raw_step)
      Raw_Vertex, Raw_KI, Raw_KII, Raw_KIII = raw_step[0], raw_step[1], raw_step[2], raw_step[3]
  
      subplot(311)
      plot(Raw_Vertex, Raw_KI, 'bo-')
      grid(True)
      title('Raw SIFs')
      ylabel('K_I')
  
      subplot(312)
      plot(Raw_Vertex, Raw_KII, 'go-')
      grid(True)
      ylabel('K_II')
  
      subplot(313)
      plot(Raw_Vertex, Raw_KIII, 'ro-')
      grid(True)
      xlabel('Vertex')
      ylabel('K_III')
  
  show()
  
  
  for mls_step in mls:
      mls_step = zip(*mls_step)
      MLS_Vertex, MLS_KI, MLS_KII, MLS_KIII = mls_step[0], mls_step[1], mls_step[2], mls_step[3]
  
      subplot(311)
      plot(MLS_Vertex, MLS_KI, 'bo-')
      grid(True)
      title('MLS SIFs')
      ylabel('K_I')
  
      subplot(312)
      plot(MLS_Vertex, MLS_KII, 'go-')
      grid(True)
      ylabel('K_II')
  
      subplot(313)
      plot(MLS_Vertex, MLS_KIII, 'ro-')
      grid(True)
      xlabel('Vertex')
      ylabel('K_III')
  
  show()
  </code></pre>