title: Retrieve past n records of panda dataframe based on column value
body: <p>I have a panda dataframe <code>df</code> below;</p>
  
  <pre><code>         pay        pay2        date
  0  209.070007  208.000000   2018-08-06
  1  207.110001  209.320007   2018-08-07
  2  207.250000  206.050003   2018-08-08
  3  208.880005  207.279999   2018-08-09
  </code></pre>
  
  <p>Given a <code>last_date</code>, I want to retrieve the past <code>n</code> rows from <code>last_date</code> onwards, including the row of <code>last_date</code> itself.</p>
  
  <p>For instance, given <code>last_date = '2018-08-08'</code> and <code>n=3</code>. The result dataframe should look like this;</p>
  
  <pre><code>         pay        pay2        date
  0  209.070007  208.000000   2018-08-06
  1  207.110001  209.320007   2018-08-07
  2  207.250000  206.050003   2018-08-08
  </code></pre>