GUI - Allow user to "draw" function. Also and GUI advice appreciated

["Julian Tam" <juliantam@gmail.com>]

I'm trying to write a program for image histogram specification.
Unfortunately, this is the first time I've ever attempted a GUI in
MatLab, so I hope it's something I can do...

Essentially, I want to have an axis, on which a user can click down
on the left hand side of the axis (or anywhere...), and draw a line
moving across the graph, thus creating a new function. I then want
to place the f(x) values in a vector.

Is this possible?

Also, can anyone suggest some MatLab GUI tutorial websites?

Thanks