It never fails! As soon as I post the question, I come up with a solution.
I created a calculated field called "DataExists" with:
IF isnull(sum([Total Cost])) then 0 else 1 end
Then, I edited the table calc "in shelf" and simply appended " * DataExists " to the end.
Works pretty good, but if anyone else has any better ideas, please share.
Glad you got it working. Here's another approach:
If you drag a Quick Table Calculation to the Measures list on the left, it will create a calculated field that does the same thing. Here, you can see why your chart interprets non-existent values as zero. It's because this is the default formula that Tableau uses for a "Difference" table calc:
The ZN() wrappers ensure that non-existent values get converted to 0's. So, just remove those…
…and then your chart will look like you want it to. (You will now see a "51 nulls" in the lower-right, but that's to be expected, so you can hide that indicator.)
I like that approach better. Thanks!