Yxy colorspace is a Chromaticity space, since its primary purpose
is to provide two coordinates that display the hue and saturation
of a color. The Y value is that of the XYZ,
while the x and y are projections of the X and Y values:

Y = Y |

x = X / (X + Y + Z) |

y = Y / (X + Y + Z) |

One of the properties of x,y values is that straight lines in XYZ space remain straight lines in x,y space.

The Chromaticity diagram based on x,y values is widely used, but is very perceptually non-linear, exagerating the green area and minimizing the apparent blue area. For this reason, the Yu'v' Chromaticity space is preferable.