title: Re-index dataframe by new range of dates
body: <p>I have a data frame containing a number of observations:</p>
  
  <pre><code>date         colour     orders
  2014-10-20   red        7
  2014-10-21   red        10
  2014-10-20   yellow     3
  </code></pre>
  
  <p>I would like to re-index the data frame and standardise the dates.</p>
  
  <pre><code>date         colour     orders
  2014-10-20   red        7
  2014-10-21   red        10
  2014-10-22   red        NaN
  2014-10-20   yellow     3
  2014-10-21   yellow     NaN
  2014-10-22   yellow     NaN
  </code></pre>
  
  <p>I though to order the data frame by <code>colour</code> and <code>date</code>, and then try to re-index it.</p>
  
  <pre><code>index = pd.date_range('20/10/2014', '22/10/2014')
  test_df = df.sort(['colour', 'date'], ascending=(True, True))
  ts = test_df.reindex(index)
  ts
  </code></pre>
  
  <p>But it returns a new data frame with the right index but all <code>NaN</code> values.</p>
  
  <pre><code>date         colour     orders
  2014-10-20   NaN        NaN
  2014-10-21   NaN        NaN
  2014-10-22   NaN        NaN
  </code></pre>