user exit for a varialble

hi gurus

i have req in which a report should accept a month as an input, and than based on that it should show actual data until than followed by plan data for the rest of the year

in technical words if user enters march as month, columns until march should have the keyfigure restricted by a characteristic (VTYPE = 10 - Actual) and for the remaining months it should retrict the keyfigures by (Vtype = 20 - Paln).

i believe this will require a Abap code that assigns the variables for Vtype a value based on the month input., can some one provide me with a sample code that does this or something similar to this.

if there is any other way to do this please let me know.

i am working on BI7

thanks