I have a problem getting my pie chart right. Let me point out right away that the chart is built by 0 dimensions and three expressions. I suspect having one dimension with the three values 'a', 'b', 'c' and one expression could solve my problem, but I would rather not go that way, if possible.
Back to the problem, you can see how, although I am hovering over 'b' with the mouse, in the pop-up label we see 'a=...'.
This discussion looked promising, as it achieves to dynamically display the result of the expression within a custom text. However I would need to notch it a step further and also have the expressions' label be displayed.