CIE xyY 3D surface plot is incorrect

[mm2t]

Hello.

Thank you for offering this library.

The figure for the CIE xyY 3D surface plot (file: visible-gamut-xyy.png) in the README, I believe is incorrect.

I am not a color scientist, but I understand that it is not a regular cross-section.
See: http://www.brucelindbloom.com/images/GamutxyY_WF.gif and http://www.brucelindbloom.com/index.html?ChromaticityGamuts.html.