Diagonal line in Numbers cells

I have a Numbers spreadsheet and want to add diagonal lines in some, not all, of the cells. I cannot find a way to do that where the line is IN the cell. Adding a line segment to the spreadsheet just overlays the spreadsheet with the line, it's not actually associated with any cell.

So, how does one format a cell to have a simple diagonal line from lower left to upper right of a cell?

EDIT: Mavericks, Numbers 3.1