title: Pandas unstack doesn't work
body: <p>Initially I had DF with 1 column of actions indexed with DatetimeIndex:</p>
  
  <pre><code>In [371]: dates
  2013-12-29 19:21:00    action1
  2013-12-29 19:21:01    action2
  2013-12-29 19:21:11    action1
  2013-12-29 19:21:13    action2
                             ...
  In [372]: dates.index
      Out[372]: 
      &lt;class 'pandas.tseries.index.DatetimeIndex'&gt;
      [2013-12-29 19:02:27, ..., 2014-01-13 16:30:31]
      Length: 108957, Freq: None, Timezone: None
  </code></pre>
  
  <p>I want to plot number of actions of certain type vs day</p>
  
  <p>So I grouped actions by date, using <code>agg</code> </p>
  
  <pre><code>grouped = dates.groupby([dates.index.to_period(freq = 'D'), 'actiontype']).agg(len)
  </code></pre>
  
  <p>Which gave me multiindexed series:</p>
  
  <pre><code>...
  2014-01-13  action1       435
              action2       2067
  ..
  2014-01-14  action1       455
              action2       1007
  ...
  </code></pre>
  
  <p>Which seems to be precisely what I need. </p>
  
  <p>But when tried <code>unstack</code> the series to get rid of the MultiIndex and plot my data, and got the error:</p>
  
  <pre><code>In [379]: grouped.unstack()
  
  ValueError: freq not specified and cannot be inferred from first element
  </code></pre>
  
  <p>What's my mistake here? Thank you.</p>