title: Pandas timeseries groupby using TimeGrouper
body: <p>I have a time series which is like this</p>
  
  <pre><code>            Time    Demand
  Date        
  2014-01-01  0:00    2899.0
  2014-01-01  0:15    2869.0
  2014-01-01  0:30    2827.0
  2014-01-01  0:45    2787.0
  2014-01-01  1:00    2724.0
  2014-01-01  1:15    2687.0
  2014-01-01  1:30    2596.0
  2014-01-01  1:45    2543.0
  2014-01-01  2:00    2483.0
  </code></pre>
  
  <p>Its is in 15 minute increments. I want the average for every hour of everyday.So i tried something like this <code>df.groupby(pd.TimeGrouper(freq='H')).mean()</code>. It didn't work out quite right because it returned mostly<code>NaNs</code>. </p>
  
  <p>Now my dataset has data like this for the whole year and I would like to calculate the mean for all the hours of all the months such that I have 24 points but the mean is for all hours of the year e.g. the first hour get the mean of the first hour for all the months. The expected output would be</p>
  
  <pre><code> 2014 00:00:00  2884.0
   2014 01:00:00  2807.0
   2014 02:00:00  2705.5
   2014 03:00:00  2569.5
   ..........
   2014 23:00:00  2557.5
  </code></pre>
  
  <p>How can I achieve this?</p>