title: How do you plot a vertical line on a time series plot in Pandas?
body: <ul>
  <li>How do you plot a vertical line (<code>vlines</code>) in a Pandas series plot?</li>
  <li>I am using Pandas to plot rolling means, etc., and would like to mark important positions with a vertical line.</li>
  <li>Is it possible to use <code>vlines</code>, or something similar, to accomplish this?</li>
  <li>In this case, the x axis is <code>datetime</code>.</li>
  </ul>