Jack,
If you do not have labview 6.1 you can still get the mouse coordinates by using a call to the windows 32 API dll. There's a library called LVutil32.zip which contains a set of llb's and a dll(see instructions to install). One of them is the winsys.llb and there's a get cursor positon.vi:
http://zone.ni.com/devzone/explprog.nsf/webmain/ebf11dd1f0feaef3862566790072a983?OpenDocument
You can compare those coordinates to the cursor coordinates on the graph to correlate them.
There's also a VI that allows you to capture a mouse double click.
http://zone.ni.com/devzone/explprog.nsf/6c163603265406328625682a006ed37d/6b8cc338382df40786256a62004fa2ec?OpenDocument
Hope it will help.
Regards
Cyril Bouton
Active LabVIEW Developper