Create a variable like Web Intelligence

Hi,

I'm trying to create a new variable in Design Studio

In my request, i have 3 columns :

Product / Volume Current Year / Volume Last Year

I want to create a new column [Growth] = (Volume Last Year]/[Volume Current Year]) -1

and use it in graphs

How can i do that in Design Studio ?!?
In Web Intelligence, i just need to add a new variable, and use it in graphs

Thx by advance