Thank you for your detailed reply. I have implemented it for the data source I will use(confidential data hence my dummy data workbook) but still have two outstanding questions, I think they are related:
1) How do you get the sort order correct. I only have option to sort by 'Data Source Order' and this produces something like the attached file.
2) From what I can see this does not 'group' the data, but rather renames it. When you hover over a 'rest' grouping it will show the details of one of the many items grouped in rest (i.e. and not the 'Rest' total). Is there another way to group the 'Rest' Category together?
Tableau Pie.png 50.6 KB
Here's an alternative, that uses a similar starting approach to the above but adds one more step of indirection that enables us to draw N+1 slices instead of a slice for every Parent, and along with that able to show more details on the mark labels, etc. The worksheet is based on this Joe Mako post: http://community.tableau.com/message/128357#128357.
I created a bar chart version as well. Note that the hidden headers are required to make the sorting work properly, since Tableau can't sort by measures that are table calculations (but it will alphanumerically sort discrete values regardless of their source).