It’s the market caps of the preferred equities issued by Strategy & Strive.
Basically they’re fixed income products backed by bitcoin. The treasury companies call it “Digital Credit”
The chart excludes Strategy’s STRE & Metaplanet’s prefs because they’re not publicly traded. Think those two would add smthn like ~$1-2B iirc
TLDR:
Close to $10B has gone into bitcoin via these fixed income products in about a year