Including summary metrics

This is just to write down a point I discussed with Thomas so we don't forget.
Because the `DRAW` clause doesn't distinguish between geom and stat and geoms/stats cannot be arbitrarily combined, we'd need to think about if we can do some degree of computing summaries.

This may be especially pertinent for some layers:
* `errorbar`
* `ribbon`
* A little bit of aggregation for `hline` and `vline`