Hi @Vincent maybe this is more a question for @tryan but in the API is there in fact actually anything more than proportional spread? Straight line, repeat, growth ... are really data entry shortcuts for leaf cells. Percent change is really just proportional spread with one simple precursor arithmetic step to target value.
Relative proportional spread seems an exception because it requires a GUI to do the selection of the relative cell coordinates. And the GUI needs some smarts beyond simple selecting a cell as not all cubes and all cells are valid. Once you have the coordinates then it is not too hard to mimic relative proportional spread with TI to copy from source to target to get the "pattern" and then follow with normal proportional spread. So I would guess even if "relative proportional spread" does not exist in any API it would still be possible to do but just needs a lot more development time as it needs a GUI. Are my assumptions on the money?