Create a new calculated field, containing the following:
When "Brand 2 Affiliated" then "Brand 2"
Then use that new calculated field instead of your old one.
Harriet, the easiest way to do this is to create a brand new calculated field and use that field as your "Brand Group" dimension.
IF CONTAINS([Brand Group],'Brand 2')
THEN 'Brand 2'
ELSE [Brand Group]
If you DO need to show the individual values for the 2 groups you've combined, you can always use the original Brand Group field (for colors, for example).
Thanks for your response! I was hoping there was a more sophisticated way than doing a calculated field but this works just as well anyway!
Normally, Tableau should aggregate on the right level though, even when using groups. have you included your original dimension somewhere in the graph (on details, or on color, eg.)? Remove/replace that with the group, and it should stack as well.