5120x2880
I'm thinking in the same terms I used to think when I was doing home audio. My boss had a set of $10K speakers. Sure, they sounded great, but then I listened to a clients $5k speakers and couldn't tell the difference between his and my bosses speakers. It was as if my ears weren't finely tuned enough to tell.
Of course if you ask my Boss about the difference, he'd take an hour to tell all the differences. To me, the lay person at the time, I couldn't tell.
It doesn't really say, but I would guess it has to do with when you start to see individual pixels.
But gee, one length (viewing distance) is in different units than the other length (screen diagonal size). Hilarious.
The X axis should go from 10 to 60, the Y axis should go from 1ft to 30ft, max.
As it is, it's not all that useful for figuring out what the density limits of a desktop monitor are. (Average desktop monitor sizes occupy maybe 1/12 of the X axis!)
Then simple trig tells us that we are maxed out when:
(distance to screen) * sin(0.3 arcminutes) = pixel size
The new iMac has a 27 inch diagonal, and 5874 pixel diagonal, so its pixel size is 0.004596 inches. Plug that in above, and you can see that you would have to be 53 inches from the screen to be maxing out your visual abilities. Anything closer (like normal) and you actually could still benefit from higher resolution.