Welcome to the forums! Here's a thread where I listed a number of posts on this topic, there are a variety of ways to go about it, all of them involve doing some padding of your data:
Thanks Jonathan; I did search around before posting, but I clearly wasn't using the right keywords.
The example showing data padding works OK (http://www.datadrivenconsulting.com/2012/10/queues-and-utilization-in-tableau-part-1-queues/) - even if, I must say, I was hoping for something a bit more "out-of-the-box" from Tableau...
One thing I can't figure out how to do with that approach is how to let the "chart consumer" filter the range of data, and still keep the "totals" correct; as the suggested approach relies on running totals, whatever date range I choose, the starting value is 0 - while, in my usecase, it should be whatever number it is on the selected start date since the beginning of time. Not sure if there is a way to do that without changing the original data source.
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Instead of using a regular filter, use a table calc filter like LOOKUP(MIN([started]),0). As you found, filters on regular dimensions act in the data source, so the data isn't available to Tableau to compute the running sum. Filters on table calculations are performed after most other calculations are computed, so they effectively hide results.
Thanks again; that solves my problem.