Hm, perhaps BAgg might be something appropriate to include? It’s a library, in Elixir, for aggregating Beeminder data points (according to aggday, kyoom, and odom.) That is, it’s a reimplementation of this one part of Beebrain, because Beeminder itself doesn’t expose the aggragated data directly.
It’s lacking in documentation at the moment, but it’s perfectly usable as a library (in short: call Bagg.aggregate_goal/1 with a the results of Beeminder’s goal API (with datapoints=true.))
In addition, I’ve exposed BAgg's functionality as an API endpoint on Altbee, as described here. That might be the easiest way to use it from languages other than Elixir (or Erlang.)