And I think it's the same in all math graphs I can remember?

Thirded. In my math courses (e.g. geometry) the x-axis always represented the horizontal axis, but (as @B4B mentioned) the reverse is common given the indexing of 2D arrays, so I went with that instead.

And I think it's the same in all math graphs I can remember?

yep ;)

And maybe I'm just operating in a different domain than the other, but x = vertical and y = horizontal really tripped me up.

it's quite usual, actually (matching the way 2D arrays are indexed)

Admittedly, this kata does have a bit going on. It's not a simple problem in a neat little package, but I did put in effort to keep the Description succinct and make the layout easy on the eyes while keeping ambiguity as minimal as possible.

In any case, congratulations on completing this 3 kyu kata; it's certainly not an easy task.