title: Using a Pandas dataframe index as values for x-axis in matplotlib plot
body: <p>I have time series in a Pandas <code>dateframe</code> with a number of columns which I'd like to plot. Is there a way to set the x-axis to always use the index from a <code>dateframe</code>?
  When I use the <code>.plot()</code> method from Pandas the x-axis is formatted correctly however I when I pass my dates and the column(s) I'd like to plot directly to matplotlib the graph doesn't plot correctly. Thanks in advance.</p>
  
  <pre><code>plt.plot(site2.index.values, site2['Cl'])
  plt.show()
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/Y3kJD.png" alt="output"></p>
  
  <p>FYI: <code>site2.index.values</code> produces this (I've cut out the middle part for brevity):</p>
  
  <pre><code>array([
      '1987-07-25T12:30:00.000000000+0200',
      '1987-07-25T16:30:00.000000000+0200',
      '2010-08-13T02:00:00.000000000+0200',
      '2010-08-31T02:00:00.000000000+0200',
      '2010-09-15T02:00:00.000000000+0200'
  ], 
  dtype='datetime64[ns]')
  </code></pre>