[OPPM V9.0 SP1] How to calculate the variation of certain Category's Value?

Hi all,

I want to know how to calculate the variation of value regarding 'Data As Of'.
I know the value can be seen in Trend Graph, but I want to handle it as Value of Category.
Any advice would be appreciated.

for example, Category 'Revenue' has following values.

Data As Of = 2013-Jan-1, Value = 100

Data As Of = 2013-Apr-1, Value = 200

Data As Of = 2013-Jul-1, Value = 250

I want to get the value that explains latest 3-months variation from 'Revenue'.

the Category would have following value.

Data As Of = 2013-Apr-1, Value = 100 ( difference between Apr and Jan)

Data As Of = 2013-Jul-1, Value = 50 (difference between Jul and Apr)