Interesting, per the screenshot below, that almost works. It does list the empty row for the "1/4 fold party..." item, but it's not a "real" row of data, meaning that I can't do % of overall total calculation and have it return 0, even with the zn function. Furthermore, I can't do a "keep only" on this item because, again, it's not a "real row" of data.
I think the real solution to this is to just use a parameter and an "if [parameter] = Central then sales else 0 end" clause and be done with this. The only downside is I would need multi-value parameters (per this idea: https://community.tableau.com/ideas/1313) to be able to do Central and West together as per this use case. I think ultimately a filter will filter out the data leaving no "real row" there that I can do anything with.
And FYI, I posed this question to Tableau Doctor at the conference and the best we could come up with is some custom SQL added to the data to create a cross-join of every possible product/region combination filled with zeros so that you then always do have the "real row" of data you need for this to all work.
I did do a somewhat interesting hack/workaround which is attached.
I basically did a union of the data to itself and did some calculations for a new "region" and a new "sales" measure. That allowed me to put the filter you see here with "- Show Blanks" as a new region option. With that unselected, it looks like it did before with the row "missing". But with it selected, it basically substitutes zeros from the original data so that you now see all rows. This isn't the best workaround though because a.) it's an odd, non-intuitive filter selection, b.) you then can't split this by region on the view without this all breaking. So I think I'll just accept that this isn't a real doable use case in Tableau (as compared to other tools) at least until there are multi-value parameters.
Shame! but yes I agree I think the only way to do it (with current functionality) would be by cross-joining every product/region combination (with customSQL or a view in the database), and then left joining the sales back on (an maybe even by date if you also require this dimension).
Out in Beta, atm, are Set Actions, which do let you have multiple selections (so would work as a multi-select parameter in this case, I think)...so hopefully this makes it out to Tableau-real very soon.
Very valid point on set actions. I saw bits of this at TC18 and it looks promising. We shall see. Thanks!