How to read a painted area on a form (really hard to describe in a line,please enter for more details)

Question

Hi,
I have a form with a control which holds the task of showing a graph,by reading (x,y)s from a file and adding them points collection of a graph. now i want to write some code which which helps me do  the following:
when i choose a rectangle area containing some part of the graph or function on the form, a pop up form should come up and  show me the Fourier transform (or anyother transform) of the graph.
the biggest problem here is that i don't know how to extract some part of graph from the control. i mean it should give me all (x,y)s that form the graph or at least two (x,y)s that are first point and end point of graph( and then i will extract other (x,y)s from the information file). how should i do this?
any help appreciated
thanks