Or, you could blur the image a little, then use the
Level curves to make the data a thin black line by squeezing the
fully-black and fully-white arrow tags close together. Make the image
indexed with one color. Finally, save the image as a .pgm or .xpm
file. (.pbm is probably ideal but it seems like GIMP can't save in
that format. Perhaps use ImageMagik's convert to go from pgm to pbm?)
The data format for such files is very simple and with a
quick read of the relevant man page, you will be all set to read the
file, dump the data into an array and then off you go. If you have
lots of pictures and would like to automate the process, you could
even write a script to handle the GIMP part non-interactively.
This has the advantage of probably being more accurate than using the
mouse for coordinates, but perhaps this is not what you wanted. If
that's the case, sorry...