title: Adding a column thats Max value of lookback x row in pandas
body: <p>My Orignal dataframe is like:</p>
  
  <pre><code>   Date  Price
  0     0    100
  1     1     10
  2     2     30
  3     3     20
  4     4     50
  </code></pre>
  
  <p>Price are float.</p>
  
  <p>I need to be able to add a column that's "Max of last x days" including today
   to the orignal dataframe .</p>
  
  <p>The result should be
  x=3</p>
  
  <pre><code>  Date   Price   Max_3Days
      0      100    N/A
      1      10     N/A
      2      30     100
      3      20     30
      4      50     50  
  </code></pre>