Just jumping on the bandwagon, I'm having the same issue here.
I suspect that it has to do with the connection session expiration to GA.
Feels like a bug to me.
Are you on 7? I used to have this problem with one dataset. 8 solved it
I'm on 8.
I submitted a support case, and they were able to reproduce the issue. The problem is now in the development cycle, so they closed my case. We just need to keep our eyes on the release notes for future updates, and a resolution will be forthcoming.
I have same issue in Tableau V8.
Every time I change my graph type, i find that the relationship between both sets of data gets separated.
Has anyone found a workaround yet? I need to maintain this relationship for one of my calculations. I am considering exporting my GA data to excel and then using that as a data source. Not sure how I ll go about that quickly though, since I want that data source to be updated every day.