I need to create a stacked waterfall chart

[danosoft]

Hi, i need to create a stacked waterfall chart with my example i send:

- I want the first and second bar are in a stack

- i want the third an fourth bar are in another stack

- fifth, sisth and seventh in another stack

how can i do that with this example i attach?

i use some different expression for every bar, so i don't know what i can use for my dimension for doing that