C.I.E. Chromaticity Diagram
Here is a very detailed dissertation on the subject of chromaticity, along with software for generating multiple version of the chart.
Chromaticity is commonly defined as the quality of color characterized by its dominant or complementary wavelength and purity taken together. This chromaticity chart plots the human tristimulus color space according to:
x = X/(X + Y + Z),
y = Y/(X + Y + Z).
Colors along the periphery of the "horseshoe" plot (spectral locus) are pure, monochromatic, saturated colors. Less saturated colors approach the center becomes (i.e. it becomes whitish or grayish). The straight line across the bottom is called the "non-spectral line of purples."