Generating multiple coordinates from a graph in .jpeg format

I need to generate random(say 3) coordinates from each line in a graph. I don't know what to start with. I don't even have the experimental data using which the graph was plotted. Let say I have this image (my graph is way more complex though). Now, I want to generate 3 coordinates (x,y) from each line (y1,y2 and toto) that satisfies the line. example: y1- (2,0),(4,2),(5,0) y2- (4,5),(6,8),(10,13) toto - (4,14),(5,15),(9,18) What would be the best way to do this?

a random graph