The CIE XY space is not perceptually uniform, and your article states "correlated color temperature is the color temperature of the point on the blackbody locus that is closest in appearance (chromaticity-wise) to the chromaticity of interest. Technically, this means the point on the blackbody locus nearest the point representing the chromaticity of interest when the blackbody locus is plotted not on the CIE-xy chromaticity diagram but rather on the CIE-uv space chromaticity diagram"
Therefore, for this type of plotting you should be using a perceptually uniform space such as CIE-uv or CIE LAB, not CIE-XY. If you want to find the shortest distance from a point to a line, you drop a perpendicular from the point to the line. With CIE-xy the lines of correlated color temperature are not perpendicular to the Planckian locus, but with a perceptually uniform space, I would think that they should be. If you want to nit pick, you should have your ducks lined up in order.
A tint of -2 in the magenta direction is very close to the Planckian locus. Of course, magenta is not on the Planckian locus, which runs in the yellow-blue direction. Really, what point are you trying to make?
CIE_Lab cannot be used as white reference is required.
The orthogonal lines are computed on CIE-uv space and then converted to CIE-xy space for plotting. That was done to save the same dyagram reference.
I confirm that your photo lights are far from Planckian locus.
I confirm that using a graycard you can correct images illuminated by non-blackbody sources.