So I should overwrite the measure field definition on the report model I'm using, then I can get the avg as a operator only for desired report ? on the other hand, I'm looking for the original measure field definition, on the models.py (ORM) but can't find it.

when I said measure field definition I really mean to say the definition of the field used as measure of the pivot table graph view that you need to group using avg, that field in your report model need to have the group_operator = 'avg'

did you test it?

Great ! It works fine, thanks a lot Axel :)

@Axel, How can I change the 'total' string (see http://prntscr.com/9eexf9) in to 'average' ?

I don't know how to do it without an extension

How is it possible to do it with an extension ?

Please create another question for that, to explain all the steps of how to do it in an answer without mix things

Done : https://www.odoo.com/fr_FR/forum/help-1/question/odoo-8-reporting-change-default-total-label-94772 Thanks a lot Axel.