If I understand correctly 'tgt' is some kind of target relating to the y values (and independent of 'Split'). If that's so, the normal way to handle this in JMP would be as meta-data relating to column y (or as a 'column property' to use JMP language).
In the attached version of your table I've added an axis property, but a spec limit property may be more appropriate. Click on the '*' next to 'y' in the Columns Panel'. Then run the saved script.