Add a "Crosshair" to the Canvas? :)

Hi, in some analysis, I really think it useful if you can
add a crosshair feature so I can visually align the position
of peaks and falls (like the stock market), and may be not
hard to include, too. Basically, if the "crosshair" checkbox
checked, a horizontal and a vertical line will appear with
the movement of the mouse.

Bao