[psd1]

It is, inasmuch as we have 3 types of cone, which is an inherent orthogonality. It is also not, inasmuch as each cone is a wavelength in the same spectrum.

Either way, you can project a volume onto a plane, which is great for communicating visual data on paper or screen.

The interesting question is "why that arc in particular"; my ignorance will shine through if I speculate.

I assume that the projection encodes something about our relative perception of each cone's band, hence the big green corner.

[carlosjobim]

It is 3 dimensional, because in our perception we see the third dimension of magentas and purples, which do not exist in physical reality on the spectrum.