title: Python: Groupby hour include null values
body: <p>Using citibike data: <a href="https://s3.amazonaws.com/tripdata/index.html" rel="nofollow">https://s3.amazonaws.com/tripdata/index.html</a></p>
  
  <pre><code>tripduration    starttime   stoptime    start_station_id    start_station_name  start_station_latitude  start_station_longitude end_station_id  end_station_name    end_station_latitude    end_station_longitude   bikeid  usertype    birth_year  gender
  461 2016-02-01 00:00:08 2016-02-01 00:07:49 480 W 53 St &amp; 10 Ave    40.766697   -73.990617  524 W 43 St &amp; 6 Ave 40.755273   -73.983169  23292   Subscriber  1966.0  1
  297 2016-02-01 00:00:56 2016-02-01 00:05:53 463 9 Ave &amp; W 16 St 40.742065   -74.004432  380 W 4 St &amp; 7 Ave S    40.734011   -74.002939  15329   Subscriber  1977.0  1  
  280 2016-02-01 00:01:00 2016-02-01 00:05:40 3134    3 Ave &amp; E 62 St 40.763126   -73.965269  3141    1 Ave &amp; E 68 St 40.765005   -73.958185  22927   Subscriber  1987.0  1
  </code></pre>
  
  <p>Using the Groupby function grouping by hour, I would like to include null values as zero. </p>
  
  <p>I used the following code:</p>
  
  <pre><code>bikes_parked = df.groupby(['end_station_name',pd.Grouper(key='stoptime',freq='H')]).size().reset_index()
  bikes_parked.rename(columns={0: 'bikes_parked'},inplace=True)
  </code></pre>
  
  <p>Which returned number of bikes parked by the hour, but for the hours that did not have data it skipped. </p>
  
  <p>Output:</p>
  
  <pre><code>    end_station_name    stoptime               bikes_parked
  0   1 Ave &amp; E 15 St     2016-02-01 00:00:00    1
  1   1 Ave &amp; E 15 St     2016-02-01 05:00:00    1
  2   1 Ave &amp; E 15 St     2016-02-01 06:00:00    3
  </code></pre>
  
  <p>I want to include stoptime 01, 02, 03, 04 with bikes_parked as 0 as well. </p>