Hi,
I hope I have understood your problem correctly.
Have a look at the following screen-shot and read the following:
- Select the cell which you wish to display/set as a diagonal header
- Right-Click the mouse button and select 'Format Cells'
- In the box which pops-up, select the 'Alignment' Tab
- Under the 'Alignment' Tab, you would see the encircled 'Text' with a line
to the right side (horizontal).
You need to move this angular upwards (or downwards) based on the
manner in which you wish to display the header diagonally.
Hope this Helps.