Thanks! I had read the documentation but wasn’t much wiser. I tried the \n for a line break but that didn’t seem to do anything.
Your post prompted me to add the date into the bullet tooltip rather than the X-axis labels. This works and to effect the line break you need to enter the text then edit the text via the dropdown text editor and add in a line break without the \n.
This solves the problem satisfactorily and I can delete the X-axis tooltip.