feat: state sum to show sum of available points

[jasonlewis]

I wanted a way to show the default state of the graph as a sum of the available points rather than the current/last point.

This adds a new sum value for state which sums the available points. This does rely on the fix within #1075 for it to work as it expects points to always be defined.

[aaaaahaaaaa]

This is actually relevant to the issue is just opened: #1095

I think your solution should be more holistics and cover all the aggregate_func types, not just sum. That feature would defo be a great addition.

[przemek-baranowski]

My understanding is that the solution should work for all aggregation functions. It fetches values from previously generated points. "sum" is just another type of state; it does not correspond with any agg func. Prove me wrong or please merge this one. I'd be happy to use it ;)