Problem solved...

Well, I still have to read up on the XYZ space, to more or less confirm my idea, but I at least solved the missing Y problem.

The grid to edit on is 2D, so the third dimension is lacking. Then after adding the slider for Y it turned out that Y actually changes the luminosity of the color in the proofing photo.

The color gets brighter or darker (again opposite of what you would expect: when Y goes down, the color becomes brighter. I suspect that has to do with the fact that the content of the edited profile is compared in the translation with the standard profile and the difference of values is taken...).

Then I thought: that third dimension, imagined on a 2D grid, is as if you're actually pushing the point away from the viewer, or pulling it back, towards the viewer.

So... when 'pulling' it towards the viewer (by increasing Y) the point in the grid (red, green, blue or white) should get larger.

And when Y decreases, the point on the grid should get smaller (the point gets further away in the imaginary third dimension).

That third dimension isn't there, but you can simulate it this way, as if you're looking into a cube.

It gives a very nice impression of the amounts of the color.

So now you can shift the point, and make it larger or smaller.

Problem solved without complicated 3D DirectX efforts...

The improved point editor, with possibility to edit Y... the bigger the point, the higher Y, the 'closer' it is... in this example blue is pushed away the most, with green being the closest...