@GED: can you explain how to create controller for graph view? If possible give one example. Thank you.

i could, but this would be quite involved. It is a non trivial task, and of course, I don't have so much time. Doing this would require adding a controller which will acts as a specialized read_group, calling read_group when possible, and aggregating the data when a functional field is not stored, then the graph view could in theory call that controller instead of readgroup. However, (1) that controller would be difficult to program, because the groupbys could be any combinaison of functional fields and normal fields, and you want to use the native read_group as much as possible, (2) I am not sure that this is a good idea performance wise. It will probably be too slow (and too cpu/memory hungry) when aggregating millions of rows.