title: Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Sum
body: <p>Data for every month is there till December. I want to get a new column as "sum" , which will sum till the previous month. Means, now it is august. So, the sum should be placed till july as shown in first entry in "Sum" column below. </p>
  
  <pre><code>Jan| Feb| Mar| Apr| May| Jun |Jul |Aug |Sum     
  21 | 28 | 26 | 31 | 54 | 67  |38  |29  |265             
  11 | 44 | 66 | 7  | 88 | 54  |90  |74  |                
  13 | 45 | 26 | 38 | 36 | 39  |67  |49  |            
  76 | 35 | 67 | 23 | 76 | 54  |35  |59  |    
  </code></pre>