The entire visible color space (the gray area below) cannnot be obtained in printing or color television display
using Y and the color difference signals (Cb/Cr). RGB can nearly duplicate the entire color space.
This is why many graphic systems use RGB.
The gray number values around the gray area are the color wavelength in nanometers.
The diagram actually is a 3D representation where we're are looking only at the x & y plane
Humans have three sets of color sensing cones in our eyes.
The y axis corresponds to human response to to Luminance (this is where the term Y comes from).
x is a mix of non-negative values from our other two cones.
z is loosely representative of our response to blue light. Our eyes are least sensitive to blue light.
For any given Y value, the XZ plane will contain all possible chromaticities at that luminance.