I'm trying to add a 2nd axis to a chart I'm creating, but the 2nd axis will not have constant intervals like in the image below. Can the letter type axis be created, and can it be done by only adding one axis?

I'm trying to add a 2nd axis to a chart I'm creating, but the 2nd axis will not have constant intervals like in the image below. Can the letter type axis be created, and can it be done by only adding one axis?
