i think that You could try this: when the mouse is on the cross point of cursor and curve
You will see the rectangular region with the limits from 0 to 1, either for x and for y dimension;
while the rectangular is visible You can move the cursor with Your mouse and
messages are sent with the coordinates of cursor in the range of the region
(default 0, 1). It depends either by the message sent, that You can edit.
I guess that You need to work on the range of the curve.
But some expert, that i am not, will explain better i hope,