title: How to use pandas_ml to make a confusion matrix with 2 classes?
body: <p>I am trying to make a confusion matrix with plotting, pandas_ml seems to have a function but it doesn't work with 2 classes.  Is there some secret option to get it working?</p>
  
  <pre><code>from pandas_ml import ConfusionMatrix
  ytrue = ['ham', 'ham', 'spam']
  ypred = ['ham', 'spam', 'spam']
  cm = ConfusionMatrix(ytrue, ypred)
  cm
  </code></pre>
  
  <p>results in</p>
  
  <pre><code>Predicted  False  True  __all__
  Actual                         
  False          0     0        0
  True           0     0        0
  __all__        0     0        0
  </code></pre>
  
  <p>This:</p>
  
  <pre><code>from pandas_ml import ConfusionMatrix
  ytrue = ['ham', 'ham', 'spam', 'third']
  ypred = ['ham', 'spam', 'spam', 'third']
  cm = ConfusionMatrix(ytrue, ypred)
  cm
  </code></pre>
  
  <p>results in</p>
  
  <pre><code>Predicted  ham  spam  third  __all__
  Actual                              
  ham          1     1      0        2
  spam         0     1      0        1
  third        0     0      1        1
  __all__      1     2      1        4
  </code></pre>