Check out this thread which talks about another option using the built-in totalize function to do an average.
Re: chart-local, have you tried using subroutines? If you move (copy/paste) your chart logic into a sub, you can have a chart that just calls a subroutine, then all those variables are “local” to the sub, and won’t appear in the big tag tree and won’t be exposed in Display. There are some limitations to subroutines, which I talk about in this post on subs in general… which also talks about averaging!
Hope that helps…
-OptoMary