I want to draw a graticule (latitude / longitude lines) on a map I'm making using GIMP (mostly). To keep it simple, I'm using a cylindrical projection, which means of course all the lines are straight lines. The trick is, the parallels are not all equally spaced and the equator is not in the center of the map. How do I draw a straight line in Gimp precisely where I need it to be?

Thanks for any ideas,