title: Pandas plotting: How to format datetimeindex?
body: <p>I am doing a barplot out of a dataframe with a 15min datetimeindex over a couple of years.
  Using this code:</p>
  <pre><code>df_Vol.resample(
      'A',how='sum'
  ).plot.bar(
      title='Sums per year',
      style='ggplot',
      alpha=0.8
  )
  </code></pre>
  <p>Unfortunately the ticks on the X-axis are now shown with the full timestamp like this: <code>2009-12-31 00:00:00</code>.</p>
  <p>I would prefer to Keep the code for plotting short, but I couldn't find an easy way to format the timestamp simply to the year (<code>2009...2016</code>) for the plot.</p>
  <p>Can someone help on this?</p>