title: Colorbar limits are not respecting set vmin/vmax in plt.contourf. How can I more explicitly set the colorbar limits?
body: <p>Getting a strange result when trying to adjust the data range when plotting using contourf</p>
  
  <pre><code>import matplotlib
  import numpy as np
  import matplotlib.cm as cm
  import matplotlib.mlab as mlab
  import matplotlib.pyplot as plt
  
  delta = 0.025
  x = np.arange(-3.0, 3.0, delta)
  y = np.arange(-2.0, 2.0, delta)
  X, Y = np.meshgrid(x, y)
  Z1 = mlab.bivariate_normal(X, Y, 1.0, 1.0, 0.0, 0.0)
  Z2 = mlab.bivariate_normal(X, Y, 1.5, 0.5, 1, 1)
  # difference of Gaussians
  Z = 10.0 * (Z2 - Z1)
  
  plt.figure()
  CS = plt.contourf(X, Y, Z, vmin = 0, vmax = 3)
  plt.title('Simplest default with labels')
  plt.colorbar()
  
  plt.show()
  </code></pre>
  
  <p>Results in this for me:
  <a href="https://i.stack.imgur.com/y5Ze2.png" rel="noreferrer"><img src="https://i.stack.imgur.com/y5Ze2.png" alt="enter image description here"></a></p>
  
  <p>It's like the colors match the vmin/vmax I set, but the number range displayed on the colorbar remains what it would be without setting vmin/vmax.</p>
  
  <p>In this case, I would like the end result to have a colorbar that ranges from 0 to 3.</p>