How to change y axis numbers in Numbers for mac to show as custom letters or words?
This admittedly is a bit of a hack but I can't find another solution that works in Numbers.

One way to make letters appear over the numeric values is to position textboxes with the letters over the values to block out the numbers below them. Just click the Text button in the toolbar, drag the textbox to the correct position, size it appropriately and set it to have a white fill colour and no border.