title: Add line to pandas plot
body: <p>Using pandas I create a plot of a time series like this:</p>
  
  <pre><code>import numpy as np
  import pandas as pd
  
  rng = pd.date_range('2016-01-01', periods=60, freq='D')
  ts = pd.Series(np.random.randn(len(rng)), index=rng)
  
  ax = ts.plot()
  ax.axhline(y=ts.mean(), xmin=-1, xmax=1, color='r', linestyle='--', lw=2)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/pIHw8.png" rel="noreferrer"><img src="https://i.stack.imgur.com/pIHw8.png" alt="enter image description here"></a></p>
  
  <p>I would like to add another horizontal line at the level of the mean using only data from February. The mean is just <code>ts.loc['2016-02']</code>, but how do I add a horizontal line at that level that doesn't go across the whole figure, but only for the dates in February?</p>