Changing Y-Axis Values to custom scale (A-Z)

I am looking to change the Y-Axis Value scale from numbers to an alphabetical scale, A-Z.

You can see my chart is using letters A-F (let's say, eventually I do want it to be through Z). In the graph below that you can see that there are only numbers on the Y-Axis, but I want it to be the same as in my chart, alphabetical (such as the text in red to the left of the graph).

See screenshot below.