GREG is often used (but far from being limited to) for plotting two dimensional graphs showing the dependence of one variable on another. A typical graph has data points, represented by special markers such as diamonds or stars, and possibly with error bars, or perhaps plotted on the same scale a theoretical model drawn as a smooth curve. The graph must be labelled with two axes to indicate the coordinates. The other major GREG application, namely contouring, also requires such a labelling of the plotting area.
The meaning of the coordinates is entirely defined by the user. For example
in an astronomical map it may be Right Ascension Offset in arcsec along the
X-axis and Declination Offset in arcsec along the Y-axis; a correlation
study between visual extinction and molecular content will have the visual
extinction Av as abscissa (no units) and 
CO column density as
ordinate (molecules per square centimeter).
Throughout this document, these coordinates will be referred as the User Coordinates. GREG maps a selected region of the User Coordinate Space onto a specified rectangle, called the Box, of the Plot Page.
All data plotting is done in User Coordinates and clipped in the Box, except for the perspective algorithm which uses a specific sequence. Other reference systems are available for annotations (see command DRAW).