## Coordinates

There are several coordinate systems in GREG two of which are the usual basic coordinate systems of any graphic package :

- the paper or Physical Coordinate System, whose units are arbitrarily defined as “centimeters” (the size of such “centimeter” may vary from one graphic support to the other, but the dimension is always that of a distance)
- the User Coordinate System, whose units may be anything from “Butterflies per Megaparsec cube” to “Molecules per Kelvin degree and per km/s”, or anything sensible like this.

Plotting data is only a way to match these two coordinate systems, and additional work such as labelling and annotation may be most conveniently referred to one or the other system. GREG handles the transformation of coordinates and allows access to each system individually.