How to dump plotted points to matrix form from figure window?

[Guido Walter Pettinari]

Hello world!

I have a function that produces a plot. I would like to dump the plotted points 
in matrix form without having to modify the function itself.

For example, imagine that my function is:
> function plot_square ( n )
>    x = [1:n];
>   plot ( x, x.*x )
> endfunction
and imagine that I cannot modify it. If I run the function I get a figure; how 
can I dump to a matrix the plotted points (that is x and x.*x) directly from 
the figure?

Thanks,

Guido