Thank you Sunny.
Yes, you're right. Actually, I tried this but difficulty was that a part of the formula has to be summed over all the dimension.
Finally, I succeeded by adding the upper dimension on witch the sum has to be done in the context and by adding it also on the TOTAL option :
Unfortunatly, this formula can not be agregated on other dimension than the one which is specified so I have to multiply the expressions to have always the one to match where I want to use it. Here is the formula for the Département graph, if I want to make a graph per Service then I have to rewrite another expression with Service instead of Département but ok, that works like that.
Of course, if anyone has a better idea, I would be happy to know it.
Thanks in advance,