XML Elves

I'm trying to find some way to display coordinate info from a path, given one coordinate. As an example, I have a roughly horizontal path going from 0, 10 to 500, 50 with several curves in between. I can display the coordinates as I run some animation along it or use my cursor, but I'd like to enter in a value for a x-coordinate in a box and see what the y-coordinate of the path is at that point, displayed somewhere else.

Does anyone have any ideas or decent examples?

Thanks for any help.