title: Create cumulative distribution histogram for a given data
body: <p>I have the following pandas dataframe <code>df</code>:</p>
  
  <pre><code>    Bin Cum_val
  0   0   0.39
  1   1   0.63
  2   2   0.76
  3   3   1.00
  </code></pre>
  
  <p>I want to plot the cumulative distribution histagram (step-wise) that would have <code>Bin</code> values in the X axis, and <code>Cum_val</code> values in the Y axis.</p>
  
  <p>I tried this approach (both with <code>cumulative=True</code> and <code>cumulative=False</code>, since my data is already cumulative), but I get <code>Cum_val</code> values on the X axis, not on Y axis: </p>
  
  <pre><code>n, bins, patches = plt.hist(x=df.ix[:,1], normed=True,
                              histtype='step', cumulative=True)
  
  plt.grid(True)
  plt.ylim(0, 1.05)
  plt.title('cumulative step')
  
  plt.show()
  </code></pre>
  
  <p>How to solve this issue?</p>