Please check the below,it will guide you
- Advance Treemap Grouping
- 10 Lessons in Treemap Design — Juice Analytics
- Tableau 201: How to Make a Tree Map | Evolytics
- http://tableaulove.tumblr.com/post/40257187402/fun-with-tableau-treemap-visualiz ations
Thanks Kumar, but those links didn't quite give me the answer I was looking for.
However, I fixed it by a fluke. I added the combined field to the Detail mark and then could add the separate fields to the Label mark.
This seems to size and order correctly, whilst allowing me to include the level of detail I want in the label/tool tip.