title: Formatting datetime xlabels in matplotlib (pandas df.plot() method)
body: <p>I can't figure out how to change the format of these x-labels. Ideally, I'd like to call <code>strftime('%Y-%m-%d')</code> on them.  I've tried things like <code>set_major_formatter</code> but was unsuccessful. </p>
  
  <pre><code>import pandas as pd
  import numpy as np
  date_range = pd.date_range('2014-01-01', '2015-01-01', freq='MS')
  df = pd.DataFrame({'foo': np.random.randint(0, 10, len(date_range))}, index=date_range)
  ax = df.plot(kind='bar')
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/15aY7.png" alt="ugly x label formats"></p>