title: Finding missing timestamp in pandas dataframe
body: <p>I have following datasets in dataframe</p>
  <pre><code>        Time_stamp           x        y
      '2012-01-01 00:00:00'   8.97    1310.03
      '2012-01-01 00:10:00'   9.91    1684.52
      '2012-01-01 00:40:00'   9.64    1532.05
      '2012-01-01 00:50:00'   11.84   1997.87
      '2012-01-01 00:60:00'   11.69   2135.76
      '2012-01-01 01:00:00'   12.14   2149.54
      '2012-01-01 01:10:00'   13.43   2056.35
      '2012-01-01 01:20:00'   9.88    1633.45
      '2012-01-01 01:30:00'   9.01    1315.85
     '2012-01-01  01:50:00'   8.33    1141.84
  </code></pre>
  <p>As you can see, the data recorded in every 10 minutes. However, there is a missing timestamp and its corresponding values, for example, <code>'2012-01-01 00:20:00'</code> and <code>'2012-01-01 00:30:00'</code>. I would like to find such missing timestamp and replace their corresponding values with <code>nan</code>. Something like this</p>
  <pre><code>     timestamp            x      y
  `'2012-01-01 00:20:00'`   nan    nan
  `'2012-01-01 00:30:00'`   nan    nan
  </code></pre>
  <p>Any idea how to do this efficiently without much of lines of codes.</p>