Inserting diagonal line into cell in Number app on Mac

If you want something that is part of the table and moves/resizes with the cell, the only way I know of is with an image fill (cell background image). As an image fill, it will not let you change the fill color and any conditional highlighting that changes the fill color will override the image fill.

Here is a quick way to get a line as an image fill:

1. Insert a diagonal line "shape" from corner to corner in a cell.
2. Select it
3. Copy
4. Select another cell
5. Paste
6. In the format sidebar, go to the Cell tab and make the image a "stretch" fit.
7. Delete the original line so you don't forget it is just a line sitting on a cell.

For additional cells, you can copy/paste from cell to cell (which also copy/pastes the cell data), you can drag the little image from the format sidebar onto other cells and set them to stretch, you can copy style/paste style, or another way that seems to work is

1. Click on a cell that has the image fill you want. This appears to queue it up in memory
2. Select cells that need that image fill (the cell from step 1 does not have to be included in the selection)
3. In the format sidebar, set the fill to image fill. It will paste in the image and it will be set to stretch (assuming you had it set to stretch in the source cell).

If you try that last method of populating the image to other cells, let me know if it works for you.