The CIE XYZ color space was contrived so that the Y parameter carries a color’s luminance information. To see it, it must be separated from the chromaticity information. This is represented by two derived parameters x and y:   The chromaticity diagram represents all of the chromaticities visible to the average person (the gamut of human vision). It is actually a flat projection of a complex shape; x and y are the horizontal and vertical coordinates, with luminance (Y) as a third axis. The curved edge of the gamut corresponds to monochromatic light (each point representing a pure hue of a single wavelength), with wavelengths listed in nanometers. The straight edge on the lower part of the gamut is called the line of purples; these colors have no counterpart in monochromatic light. Less saturated colors appear in the interior of the figure with white at the center. When you click the red, green, and blue links to the left, the triangle formed by the cursor positions represents the gamut of the sRGB color space. https://en.wikipedia.org/wiki/CIE_1931_color_space
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