It will really depend on the Viz you are trying to create (if you can share a screen-shot?), as I don't know if you have this as a table, bar-chart...etc.
but in general...if you make the [above/below criteria] a blue-pill (discrete) and bring in to the left of the field you want to sort, that will force a sort (by default ascending, either numerically or alphabetically). You can then hide the header, so the end result is just the sort.
Hopefully that makes sense, but if not, can you post an image (or even better a mockup in a .twbx) and I can take a look if that is the best method.
Hi Simon - thanks ever so much for this.
Its exactly what I needed.