title: how to make vertical lines touch the boundaries of the chart?
body: <p>I would like the vertical lines I create with <code>ax.vlines</code> to touch the upper and lower bounds of my chart, in the same manner as <code>axvline</code> does.</p>
  
  <p>Consider this simple example:</p>
  
  <pre><code>import pandas as pd
  import matplotlib.pyplot as plt
  
  df = pd.DataFrame({'time' : [pd.to_datetime('2016-01-02'), pd.to_datetime('2016-01-03'), pd.to_datetime('2016-01-04')],
                     'value1' : [1, 2, 3],
                     'value2' : [10, 20, 30]})
  
  df.set_index('time', inplace = True)
  
  df
  Out[95]: 
              value1  value2
  time                      
  2016-01-02       1      10
  2016-01-03       2      20
  2016-01-04       3      30
  </code></pre>
  
  <p>Now the chart is: </p>
  
  <pre><code>fig, ax = plt.subplots(figsize=(30, 15))
  
  ax.plot(df.index, df["value1"], color = 'black')
  
  ax2 = ax.twinx()
  ax2.plot(df.index, df["value2"], color = 'red')
  
  #axvline stretches nicely
  ax.axvline( pd.to_datetime('2016-01-04'), color = 'red',  alpha = 1,  linestyle = '--')
  
  #vlines stops before touching the upper and lower boundaries
  ymin, ymax = ax2.get_ylim()
  ax.vlines([pd.to_datetime('2016-01-02'), pd.to_datetime('2016-01-03')],  ymin = ymin, ymax = ymax, color = 'blue', linestyle = '-')
  </code></pre>
  
  <p>gives :</p>
  
  <p><a href="https://i.stack.imgur.com/QRutV.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/QRutV.png" alt="enter image description here"></a></p>
  
  <p>You can see those blue lines remain in the air, while the nice red vertical line stretches completely. How can I achieve that?</p>
  
  <p>Thanks!</p>