I'm not sure I understand this. The Data Filter is used to select columns of a table - so I don't understand where this row number is that is being used to generated a reference line.
But lets say that I have a graph with a reference line, and as a result of some "event" (selection from a data filter?) I want to update the graph. IF I can react to the event, then I can REMOVE the reference line and ADD a new one (add ref line and remove ref line are messages that can be sent to the axis display box).
The simplest way to react to events is to let JMP handle it by using automatic recalc - but that will pick up changes to the exclude and hidden row states - both of which can be controlled through the data filter - but will not pick up this "thing" you want to use for your reference line level. Using a global variable wont work because the value not the variable gets encoded. So you need to handle the event yourself which you can do by using the Make Row State Handler data table message.
-Dave