aaah, so you're trying to control a multislider with an xy input. Got it.
The biggest issue in solving this is that you need to upsample your input, because you have 512 sliders, and only 128 steps of resolution on your midi input.
I solved this by doing extra draws with uzi. There may be more elegant solutions, but this seems to work - just don't do it with the pictslider itself, it doesn't output all its values, it would seem.