GUI: locating mouse on graph indicator

[kamocat]

I'm working on another non-FRC project. The idea is that you can draw a waveform on a graph, and hear it played. However, instead of drawing a continuous curve, I figured it'd be easier to just select some points, and spline fit between them.
So, part of that involves converting the location of the mouse into the point on the graph, and that's where I'm having some trouble. It mostly works, but there's some scaling and offset issues.
I've attached the subVI and the testing VI.

NOTE:
If you do not understand control references, event structures, and property nodes, the code may be quite confusing.

EDIT: I forgot the attachement. Thanks Tanner.