Bearer and joist sizes come from the Supplement Span Tables for the case (single or upper storey loadbearing walls by FLW, lower-storey loads, floor loads only, decks by height above ground), with limits on cantilevers and offsets under loadbearing walls, maximum roof point loads, trimmer sizing (over 3000 mm designed as bearers) and connection of trimmers and curtailed joists over 1000 mm with proper connectors; lower-storey joists in two-storey houses may not carry loadbearing walls within their span.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.