title: matplotlib plot_date() add vertical line at specified date
body: <p>My data is stored as a pandas dataframe. I have created a chart of date (object format) vs. percentile (int64 format)  using the plot_date() function in matplotlib and would like to add some vertical lines at pre-specified dates. </p>
  
  <p>I have managed to add point markers at the pre-specified dates, but can't seem to find a way to make them into vertical lines.</p>
  
  <p>I've found various answers on SO/SE about how to add vertical lines to a plot(), but am having trouble converting my data to a format that can be used by plot(), hence why I have used plot_date().</p>
  
  <p>Sample data:</p>
  
  <pre><code>date       percentile
  2012-05-30  3
  2014-11-25  60
  2012-06-15  38
  2013-07-18  16
  </code></pre>
  
  <p>My code to plot the chart is as below:</p>
  
  <pre><code>x_data = data["date"]
  y_data = data["percentile"]
  
  plt.figure()
  plt.plot()
  
  #create a scatter chart of dates vs. percentile
  plt.plot_date(x = x_data, y = y_data)
  
  #now add a marker at prespecified dates - ideally this would be a vertical line
  plt.plot_date(x = '2012-09-21', y = 0)
  
  plt.savefig("percentile_plot.png")
  plt.close()
  </code></pre>
  
  <p>Unfortunately I can't provide an image of the current output as the code is on a terminal with no web access.</p>
  
  <p>Any help is greatly appreciated - also in terms of how I've asked the question as I am quite new to SO / SE.</p>
  
  <p>Thank you.</p>