title: How to create a pandas DatetimeIndex with year as frequency?
body: <p>Using the <code>pandas.date_range(startdate, periods=n, freq=f)</code> function you can create a range of pandas <code>Timestamp</code> objects where the <code>freq</code> optional paramter denotes the frequency (second, minute, hour, day...) in the range. </p>
  
  <p>The <a href="http://pandas.pydata.org/pandas-docs/stable/generated/pandas.date_range.html" rel="noreferrer">documentation</a> does not mention the literals that are expected to be passed in, but after a few minutes you can easily find most of them.</p>
  
  <ul>
  <li>'s' : second </li>
  <li>'min' : minute </li>
  <li>'H' : hour</li>
  <li>'D' : day </li>
  <li>'w' : week</li>
  <li>'m' : month</li>
  </ul>
  
  <p>However, none of 'y', 'Y', 'yr', etc. create dates with year as frequency.
  Does anybody know what to pass in, or if it is possible at all? </p>