By default, a GDI+ function like DrawLine uses a coordinate system where the origin is in the top left corner and the units are pixels.

So DrawLine(0, 0, 640, 480) would draw a diagonal line from the top left to the bottom right of a 640x480 screen. How do I set up a Graphics object so that the origin is in the center and both axes have a range of [-1, 1]?