graph a sum from different graphs

Hello,

I'm sorry for asking you this question again but I really can't figure how to graph a sum of values coming from other graphes. Basically I want to make a graph of the total bandwidth consumed by a pool of servers. For this I would like to use the values coming from the ifstat graph of each server. What will be the best solution ?

As I said I asked this question before and got answer from two people but I was not able to have the proposed solution working. So if you have other ideas you're very welcome !

I thank you very much for your answers.

Sincerly,

ThomaS