title: Pandas plot cumulative sum of counters over time
body: <p>I have a following dataframe:</p>
  
  <pre><code>    Joined      User ID
  0   2017-08-19  user 182737081
  1   2017-05-07  user 227151009
  2   2017-11-29  user 227306568
  3   2016-05-22  user 13661634
  4   2017-01-23  user 220545735
  </code></pre>
  
  <p>I'm trying to figure out how to plot user growth over time. I figured the best way is to plot a cumulative sum. I put together a simple code:</p>
  
  <pre><code>tmp = members[['Joined']].copy()
  tmp['count'] = 1
  tmp.set_index('Joined', inplace=True)
  </code></pre>
  
  <p>This produces the following <code>cumsum</code>:</p>
  
  <pre><code>            count
  Joined  
  2017-08-19  1
  2017-05-07  2
  2017-11-29  3
  2016-05-22  4
  2017-01-23  5
  </code></pre>
  
  <p>Now when I try to plot this using <code>tmp.plot()</code> I get something super weird like this, uh:</p>
  
  <p><a href="https://i.stack.imgur.com/JoqRA.png" rel="noreferrer"><img src="https://i.stack.imgur.com/JoqRA.png" alt="cumulative sum as plotted by pandas"></a></p>
  
  <ol>
  <li>I genuinely have no idea what is this plot actually displaying (this looks like some kind of cumulative delta trend line?)</li>
  <li>How do I plot cumulative user growth over time </li>
  </ol>
  
  <p>The version of pandas I'm using: <code>pandas (0.20.3)</code></p>
  
  <p>In case you are curious whether the length of the series is the same as the highest count:</p>
  
  <pre><code>tmp.cumsum().max() == len(tmp)
  
  count  True
  dtype: bool
  </code></pre>