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