title: Pandas: Count Unique Values after Resample
body: <p>I'm just getting started with Pandas and am trying to combine: Grouping my data by date, and counting the unique values in each group.</p>
  
  <p>Here's what my data looks like:</p>
  
  <pre><code>                  User, Type
  Datetime
  2014-04-15 11:00:00, A, New
  2014-04-15 12:00:00, B, Returning
  2014-04-15 13:00:00, C, New
  2014-04-20 14:00:00, D, New
  2014-04-20 15:00:00, B, Returning
  2014-04-20 16:00:00, B, Returning
  2014-04-20 17:00:00, D, Returning
  </code></pre>
  
  <p>And here's what I would like to get to: Resample the datetime index to the day (which I can do), and also count the unique users for each day.
  I'm not interested in the 'Type' column yet.</p>
  
  <pre><code>Day, Unique Users
  2014-04-15, 3
  2014-04-20, 2
  </code></pre>
  
  <p>I'm trying <code>df.user.resample('D', how='count').unique</code> but it doesn't seem to give me the right answer.</p>