title: pandas dataframe resample per day without date time index
body: <p>I have a dataframe in pandas of the following form:</p>
  
  <pre><code>      timestamps         light
  7   2004-02-28 00:58:45 150.88
  26  2004-02-28 00:59:45 143.52
  34  2004-02-28 01:00:45 150.88
  42  2004-02-28 01:01:15 150.88
  59  2004-02-28 01:02:15 150.88
  </code></pre>
  
  <p>Here note that the index is not the timestamps column. But I want to resample (or bin the data somehow) to reflect the average value of the light column per minute , hour, day etc.. I have looked into the <code>resample</code> method that pandas offers and it requires the dataframe to have a datatime index for the method to work (unless I've misunderstood this). </p>
  
  <ol>
  <li><p>So my first question is, can I re-index the dataframe to have timestamps as the index (note that not each row has a unique timestamp and for each timestamp, there are about 30 rows with the same timestamp,each representing a sensor).</p></li>
  <li><p>If not, is there some other way to possibly achieve another dataframe which has the average value of light per hour , per day , per month etc..?</p></li>
  </ol>
  
  <p>Any help would be appreciated.</p>