Dominant wavelength
The circumferential coordinate is the dominant wavelength, which is analogous to hue of the HSV color space.
The radial coordinate is the purity, which is analogous to saturation of the HSV color space.
[1] When the chromaticity lies within the triangle with vertices at the white point, extreme spectral violet (360 nm), and extreme spectral red (780 nm), the dominant wavelength is indeterminate because the half straight line that passes through the white point and that chromaticity point intersects the limit of the visible gamut in the line of purples instead of the spectral locus.
The colors on the line of purples cannot be defined by wavelength because they do not represent monochromatic light.
These light sources are also often described by their peak wavelength—the wavelength of highest radiometric spectral flux (highest peak in the power spectrum)—but the dominant wavelength is a photometric quantity, and therefore intuitively conveys what color the light will appear without relying on inexact color naming.
The "x" marks the color in question. For the white point indicated, the dominant wavelength for "x" is on the nearer perimeter, around 600 nm, while the complementary wavelength is opposite, around 485 nm. Intuitively, the dominant wavelength of "x" corresponds to the primary hue of "x".