That would be {*;y}(x) but you are right, it’s doable with trees only, but you need to build a level-2 datatree.
Since you want to associate all items x for each plane y, you need to graft the planes Y. That’s the basic rule of GH.
Now the problem if you do that is that you end up with two trees of different levels. You can’t graft X, for two reasons : 1/ you don’t want to split the items in several branches and 2/ you’d end up with a different structure anyway. You have to duplicate the x branches as many times as there are y planes.
You therefore need to Graft Parallel (from Elefront plugin).
DataTreeQuestion.gh (23.1 KB)
Also doable with native components but so tedious I don’t see the point.