The plane diagram produced by plotting one of the three chromaticity coordinates (X, Y, Z) against another. The most common diagram is the CIE (X, Y) diagram, which is plotted in rectangular coordinates.
A diagram that represents the unit plane (the plane defined by the equation X+Y+Z=1) in a tristimulus space. The location of a stimulus with a particular set of tristimulus values on a chromaticity diagram represents its direction from the origin of the space ignoring its distance. The chromaticity diagram is often used as a convenient approximation to a constant brightness plane in the tristimulus space.