title: Pandas graphing a timeseries, with vertical lines at selected dates
body: <p>Consider this timeseries, the cumulative number of edits in a Wikipedia category. </p>
  
  <pre><code>In [555]:
  cum_edits.head()
  Out[555]:
  2001-08-31 23:37:28    1
  2001-09-01 05:09:28    2
  2001-09-18 10:01:17    3
  2001-10-27 06:52:45    4
  2001-10-27 07:01:45    5
  Name: edits, dtype: int64
  In [565]:
  cum_edits.tail()
  Out[565]:
  2014-01-29 16:05:15    53254
  2014-01-29 16:07:09    53255
  2014-01-29 16:11:43    53256
  2014-01-29 18:09:44    53257
  2014-01-29 18:12:09    53258
  Name: edits, dtype: int64
  </code></pre>
  
  <p>I have am to graph this like so:</p>
  
  <pre><code>In [567]:
  
  cum_edits.plot()
  
  Out[567]:
  
  &lt;matplotlib.axes.AxesSubplot at 0x1359c810&gt;
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/Zg91t.png" alt="cummulative edits"></p>
  
  <p>I would like to plot also vertical lines, after every <code>total_edits/n ; e.g. n=10</code> edits. I calculate these easily.</p>
  
  <pre><code>In [568]:
  
  dates
  
  Out[568]:
  
  [Timestamp('2006-06-04 04:46:22', tz=None),
   Timestamp('2007-01-28 23:53:02', tz=None),
   Timestamp('2007-09-16 10:52:02', tz=None),
   Timestamp('2008-04-28 21:20:40', tz=None),
   Timestamp('2009-04-12 22:07:13', tz=None),
   Timestamp('2010-04-09 18:45:37', tz=None),
   Timestamp('2011-03-28 23:38:12', tz=None),
   Timestamp('2012-05-24 13:44:35', tz=None),
   Timestamp('2013-03-05 17:57:29', tz=None),
   Timestamp('2014-01-29 16:05:15', tz=None)]
  </code></pre>
  
  <p>Normally one can use <code>axvline()</code> although I encounter two problems. Even if I call <code>plt.axvline(x=0.5, color='r')</code> just to produce an arbitrary line, I do not see it on top of the pandas plot. I am using IPython with <code>%pylab inline</code> by the way. And secondly, I do not now how to translate the dates into x position that are being used in <code>cum_edits.plot()</code> since the translation is invisible to me. Should I go about producing these vertical lines?</p>