Hi,
Indeed @Mark_Bailey I had a look at the documentation. But I realize that I missed one page which provides part of the answer :
"When a continuous variable is added to a grouping zone, Graph Builder uses quantiles of the data to divide the variable into five groups."
I wanted to know if there is a way to change this "quantile rule" to create intervals with constant bin. If not, I will follow the suggestion of @txnelson and create transformed variables but it can take a while if we need to redo it each time...
Well, I think I will add a wish to the wish list if there is no other loophole