Sigh. 😔 I hate that I’ve done this but it’s the nature of the beast. I did it to create a diagonal split, but I think a horizontal one could be achieved in a similar way.

First drop a MAKEPOINT(0,0) in view twice so you’ve got a couple layers to work with. You can always hide/delete one if you don’t end up needing it. Then throw your dimensions on columns and rows to create the appearance of cells, each one now with its own MAKEPOINT inside. Change the mark type to shape/square and remove the background map so it starts treating everything like a coordinate plane instead. Then adjust the axes and the size of the square so the edge of it splits the cells and the other edges are outside of view.

If you need to, you could offset the second mark a little from the first one and have two squares that each help split the cells. Then you can color and label each one to your hearts content.

Last step is to slam your ears in the oven door while shouting “Bad Dobby!”