title: Draw a cumulative chart from a pandas dataframe?
body: <p>I have a dataframe as follows:</p>
  
  <pre><code>df = pd.DataFrame({'cost_saving': [10, 10, 20, 40, 60, 60],
                     'id': ['a', 'b', 'c', 'd', 'e', 'f']})
  </code></pre>
  
  <p>How can I draw a cumulative chart of the savings?</p>
  
  <p>I'm thinking of a line chart with number of items on the x-axis, and total savings on the y-axis. </p>
  
  <p>The chart should show that the bulk of the savings come from a few items.</p>
  
  <p>I've tried: </p>
  
  <pre><code>dftest['cost_saving'].plot(drawstyle='steps')
  </code></pre>
  
  <p>but it doesn't plot the cumulative values.  </p>
  
  <p>Thanks for any help!</p>