I'm seeking the same feature/function.
I've come across this thread - Possible to add a trend line to a scatter plot in QlikSense?
which leads to another thread and a possible workbook with a formula that I'm trying to get my head around now.
Bottomline - it seems possible, but as QV has been doing this for years (above link takes you to a 2014 thread) but the 'self service' application doesn't.
Adding a reply just so you know others are interested as well.