Currently it's not available, but it's in our vision to enable the user to do small multiples (I actually prefer that name compared to trellis, honor to Tufte). From my thoughts it probably won't only be a setting on the chart, but also something that you'll be using during analysis. Too often a line chart may still look like "spaghetti" after the selection has been narrowed down, or the amount of permutations for a scatter plot takes ages to go through looking for relationships in your data.
And as you say it's extremely powerful.
by when can we expect to have small multiples available in Qlik Sense.. in one of the discussion dated 2014 it was mentioned it is on road map.. please can you provide some ETA