title: How to group pandas DataFrame entries by date in a non-unique column
body: <p>A Pandas <code>DataFrame</code> contains column named <code>"date"</code> that contains non-unique <code>datetime</code> values. 
  I can group the lines in this frame using:</p>
  
  <pre><code>data.groupby(data['date'])
  </code></pre>
  
  <p>However, this splits the data by the <code>datetime</code> values. I would like to group these data by the year stored in the "date" column. <a href="http://wesmckinney.com/blog/?p=125" rel="noreferrer">This page</a> shows how to group by year in cases where the time stamp is used as an index, which is not true in my case.</p>
  
  <p>How do I achieve this grouping?</p>