How do I split a cell diagonally?

The appearance of a split diagonal cell is just the formatting - the cell itself won't splt into 2, but you can have a line run through it.

If it is 'really' important for the sake of appearance, you could have 4 cells (arranged like a square) and have the top-right and bottom-left cells have the line go through them, then have data in the top-left and bottom-right cells.

You can't.

Peter