Market cap relative to the Thermocap (everything miners have earned since the beginning). Measures whether the market is rich relative to the cumulative security effort that produced those bitcoins.
Best viewed on a LOG scale (early years crush the axis). High = market rich relative to the accumulated security cost (past top zones); low = cheap (near bottoms). Absolute thresholds compress cycle after cycle: read it against ITS OWN history, never as a law. A slow, very-long-term indicator.