title: How to generate a new rainbow colormap using matplotlib?
body: <p>I need to plot an image filled with geophysical values ranging from 0.0 to 1.0 using a rainbow colormap.</p>
  
  <p>I tried the existing rainbow colormap from matplotlib but I am not fully satisfied with it:</p>
  
  <pre><code>from matplotlib import pyplot as plt
  import numpy as np
  
  plt.pcolor(np.random.rand(10,10),cmap='rainbow')
  plt.colorbar()
  plt.show()
  </code></pre>
  
  <p>How do I create a colormap that ranges from black for a value of 0.0 and then gradually shows the following colors: violet, blue, cyan, green, yellow and finally red for a value of 1.0?</p>