CIE chromaticity diagrams are usually exhibited in a shape of a hoof. Why are the single waves spread on the ellipse side instead of the straight?
The shape of the chromaticity diagram is an artifact of the shapes of the XYZ color matching functions and the projective transformation to xy. The curved part represents the gradually changing responsivities of the three cone types in the visual system as we sweep through the spectrum. It is curved due to the overlapping sensitivities. Near the long-wavelength end of the spectrum it becomes a straight line since only two visual responses are active for those wavelengths and not three. The purple-line (the straight line connecting the red and blue end of the spectrum) is simply derived from the additive mixture of the two ends of the spectrum, which is visible and has to fall on a straight line due to the laws of additive color mixture (Grassmann’s Laws).
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