writing plot coordinates to a file

[S. N. <se...@ma...>]

I would like to know if it is possible to write "mouse-clicked coordinates" to a
file.

For instance, the command 
 plot sin(x)

produces a plot of the sine function.

Clicking with the middle mouse button on the canvas makes the coordinates of the
clicked points visible.  Is there a way to get these coordinates to be written
to a file?  

If not, I am willing to add a small patch doing this (for my own use until it is
well-tested), but I need to know where in the source code the "mouse-cliced
coordinates" are to be found.  

Thanks for any hints.

-sen