Usually the easiest way for simple cases is to first create the graph with JMP's interactive tools:
See if you can find the setting and make modifications and get the script from red triangle menu:
Control Chart Builder(
K Sigma(2.5),
Variables(Subgroup(:Sample), Y(:Weight)),
Chart(Position(1), Limits(Show Lower Limit(0))),
Chart(Position(2), Limits(Show Lower Limit(0)))
)
and see if the option appears in the script provided by JMP.
-Jarmo