Change Y axis in clustered column chart, from values to text

[Nando1988]

I have a clustered column chart, with values ranging from 0-10. And I am currently showing integer values in the y axis, but I wish to show rankings rather than values. so for 0-2.5, D, from >2.5 and<=5, C, from >5 and <=7.5, B, and >7.5 A. I would like to show these rankings in the y axis, instead of the numerical values that currently appear, but I don't know how.
Can someone please help me?
Thanks in advance.