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.

It would be nice to have this feature efficiently across backends.
There is an example which shows you how to do this for an arbitrary

but it can be slow because it redraws the entire figure every time you
move the cursor. This is a general problem with animations in
matplotlib that we are thinking about - see for example .

There is also an example showing how to do this for WX/WXAgg (if
memory serves, you are using wx because it supports chinese
characters). This uses WX to do the cursoring, which is much more
efficient because you don't have to redraw the figure

When time permits, it would be nice to add cursoring on each backend
with native drawing of the cursor for efficiency...