Hi @josecampos,
I must say, the advice Andreas posted, does seem to fit your case “like a glove”, if
I might say so 
even if - as you say - you don’t want to “mess” with RRDs.
It looks doable though, and the URL he added, explains everything quite well.
Perhaps two other notes, that may be of interest to you:
-
In CEE you can “combine” graphs, and do calculations on them. If you do have CEE, this seems like a simple way to get this done.
-
I found this “perfcalc” plugin in the MK exchange which has been written by this awesome guy called… rsander (!) in MK exchange… (Hmm… Coincidence?
) Haven’t tested it, but could work for you. There is a note though, that it doesn’t work with 1.6 but perhaps you are still on 1.5.
Thomas
EDIT Corrected omitted word.