A crosshair which tracks a displayed curve

[MrGarbage]

Hi,
I need to implement the following functionality and am having trouble getting
started.

With a qwtplot having focus hitting a 'c' on the keyboard will cause a red cross
hair to appear in the graph. Using the mouse or the arrow keys will cause the
intersection point of the red crosshairs to follow the displayed curve.

I can get the red crosshair to appear when I click in the plot, but it disappears
as soon as I release the mouse.

Having the crosshair follow the curve - I have no idea...

Can someone point me in the right direction?


Thanks!
Mark