How to get pixel values along a line?

["Fredrik Persson" <fredrik p persson gmail com>]

I writing a gtk application where I need to do the following; let the user mark two pixels in a drawingarea (as you would if you were to draw a line in, for example, the gimp) and then, somehow, have the pixel values along that line be returned to me in some form. The goal is to present a graph to the user with those pixel values. 

The problem at hand is; how do I get those pixel values?

(And I'm writing this in pyhon, so if someone wants to provide code examples in python, that would be great.)

TIA!

/Fredrik Persson