Question on getting previous month values

[#1]

I have the following data :

Planned	Actual	Month
1000	800	jan 2011
1200	1100	feb 2011
1300	1400	mar 2011
1100	900	apr 2011
1400	1600	may 2011

I want to display the cumulative % values (cumulative actual / cumulative planned), as in :

jan = 800/1000

feb = (800+1100) / (1000+1200)

mar = (800+1100+1400) / (1000+1200+1300)

and so on...

The table must also consider different years, and the user is able to filter out those months, and data must be displayed like this :

	jan 2011	feb 2011	mar 2011	apr 2011	may 2011
%	80%	86%	94%	91%	96%

Any tips ?
I thought of using set analysis in a way that I'd sum jan + feb on feb, then jan + feb + mar on mar, but that doesn't look elegant enough...

[#2]

The best way is to load cummulative value for each month while loading the data (in load script).  The table will have Planned, Actual, Month, cummulative Planned and Cummulative Actual.   You can initialize this for each year also.

In the result table cummulative value are available for calculation.