Trailing twelve month sum

Trailing twelve month sum

Zoho,

I have been unsuccessful at creating the proper query or table to get a trailing twelve month sum for financial reports.  Below is an example of the solution in Excel.  I am trying to create a line chart report that shows my trailing twelve month billing by month. 


Month       Billings $         T12M Sum of Billings for Chart
Jan-08      $50,000         
Feb-08      $50,000         
Mar-08      $50,000         
Apr-08      $50,000         
May-08      $50,000         
Jun-08      $50,000         
Jul-08      $50,000         
Aug-08      $50,000         
Sept-08      $50,000         
Oct-08      $50,000         
Nov-08      $50,000         
Dec-08      $50,000         $600,000
Jan-09      $10,000          $560,000
Feb-09      $20,000          $530,000
Mar-09      $50,000          $530,000


Column "T12M..." is the formula column that sums the current month plus the previous eleven.  Please provide me with the proper solution so this data can generate a table from which I can create a report. 

Best,

Don