Visual details expansion

Hi, struggling to understand which way to go here. 

I have a pie chart here that shows distribution of NPT value by Equipment failure type. 

I would like to show what all these abbrevaiations mean (EN, N, E). So I thought I would create a colunm/measure and add it into the Details field of the pie chart. So the explanation would appear in the black box when I hover the mouse over the pic chart.

So I guess I would need to create an extra column that assigns to each of N, EN, E values in the current column respective values of Npt, Equipment Npt, Equipment. What would be the formula?

And should I create a custom column in Query or should I go for a measure/column in the report view?

Thanks in advance!