title: AttributeError while adding colorbar in matplotlib
body: <p>The following code fails to run on Python 2.5.4:</p>
  
  <pre><code>from matplotlib import pylab as pl
  import numpy as np
  
  data = np.random.rand(6,6)
  fig = pl.figure(1)
  fig.clf()
  ax = fig.add_subplot(1,1,1)
  ax.imshow(data, interpolation='nearest', vmin=0.5, vmax=0.99)
  pl.colorbar()
  
  pl.show()
  </code></pre>
  
  <p>The error message is </p>
  
  <pre><code>C:\temp&gt;python z.py
  Traceback (most recent call last):
    File "z.py", line 10, in &lt;module&gt;
      pl.colorbar()
    File "C:\Python25\lib\site-packages\matplotlib\pyplot.py", line 1369, in colorbar
      ret = gcf().colorbar(mappable, cax = cax, ax=ax, **kw)
    File "C:\Python25\lib\site-packages\matplotlib\figure.py", line 1046, in colorbar
      cb = cbar.Colorbar(cax, mappable, **kw)
    File "C:\Python25\lib\site-packages\matplotlib\colorbar.py", line 622, in __init__
      mappable.autoscale_None() # Ensure mappable.norm.vmin, vmax
  AttributeError: 'NoneType' object has no attribute 'autoscale_None'
  </code></pre>
  
  <p>How can I add colorbar to this code?</p>
  
  <p>Following is the interpreter information:</p>
  
  <pre><code>Python 2.5.4 (r254:67916, Dec 23 2008, 15:10:54) [MSC v.1310 32 bit (Intel)] on win32
  Type "help", "copyright", "credits" or "license" for more information.
  &gt;&gt;&gt;
  </code></pre>