title: Adding vertical line to Date formatted time-series in matplotlib
body: <p>I am trying to add a red vertical line to a plot of a timeseries where the x-axis is formatted as %Y-%m-%d.  The date at which I would like to add the line is 2013-05-14.  Simply adding the line before "plt.show()":</p>
  
  <pre><code>plt.axvline(x=2013-05-14)
  </code></pre>
  
  <p>or:</p>
  
  <pre><code>plt.axvline(x='2013-05-14')
  </code></pre>
  
  <p>returns the error:</p>
  
  <pre><code>RuntimeError: RRuleLocator estimated to generate 23972 ticks from 0044-05-12 23:59:59.999990+00:00 to 2013-06-07 00:00:00.000010+00:00: exceeds Locator.MAXTICKS * 2 (2000) 
  </code></pre>
  
  <p>Here is the function, which works well as it is:</p>
  
  <pre><code> def time_series(self):
      fig = plt.figure(figsize=(20, 20), frameon = False)
      ax1 = fig.add_subplot(3, 1, 1)
  
  
      d_dates, d_flux_n2o, d_sem_n2o = np.loadtxt('%stime_series/dynamic.csv' %self.dir['r_directory'],  delimiter = ',', unpack=True, converters={0: mdates.strpdate2num('%Y-%m-%d')})
  
      ax1.set_xlabel('Date')
      ax1.xaxis.set_major_formatter(mdates.DateFormatter('%Y-%m-%d'))
      ax1.xaxis.set_major_locator(mdates.MonthLocator())
      ax1.xaxis.set_minor_locator(mdates.DayLocator())
      plt.gcf().autofmt_xdate()
  
      ax1.errorbar(d_dates, d_flux_n2o, yerr=d_sem_n2o, fmt="y-", linewidth=1.5, label = 'Biodynamic')
      ax1.legend(loc = 0)
  
      plt.show()
  </code></pre>