In any expression you can ignore any selections and show the totals for the entire data set using Set Analysis. The prefix '1' can be used to specify the whole set, and it can be used on it's own without any qualifier, so you could have this as an expression:
sum({1}Value)
As well as giving you a total value, regardless of selections this will also cause all dimensions to show.
You need to ensure you make users aware that the expression isn't based on the selection - perhaps by means of a text box explaining this.
Hope that answers your question - if not please elaborate on what your question was.
Seems like you've deleted some posts and the responses attached, which is not a good idea, since it breaks the history of a discussion and confuses not only the ones that already took part.
I am reattaching the modified document. I haven't understood your latest comments (now deleted 😞 ), so if there are any open issues, please repost.
