Jeff is replying and I know his answer will be awesome.
The “PB” way is to do that test during the Render, as X changes from 0 to 1… (And with scaling and shifting, you can make -1 to 1 span across 0 to 1… (Example: multiply X (or Y, in your case) by 2, and now it’s 0 to 2… And then subtract 1 and it’s -1 to 1…)
That said you can also look at my post Waves upon waves for a different way to do it.