Thanks Tom, that looks to be exactly what I'm looking for.
Out of interest, I'm using Tableau Public at present, would I be better of going to Desktop and creating extracts, would that make this a more efficient process?
I'm not sure if you can Union in Public, that's a good question.
However given that you're working with sales figures, surely you don't want to publish them on Tableau Public?
I have the option to Union on public. I won't end up publishing these to Tableau Public but am using it for the moment to build some charts offline, basically I'm leaving in the unsaved "Book 1" format.
I'll most likely be purchasing a Desktop version in the near future.