You would have to look at that graph starting before 1971 when Nixon took us off the gold standard. when he did that it allowed the US to boroow more money based on nothing. The myth is that "More money has been invested in the market over time", when in fact more debt has been invested in the market.

I'm not sure that's exactly accurate.

The US was borrowing based on nothing and eventually people figured that out and wanted to trade the nothing for gold which prompted the US to "fix the glitch" and stop gold redemption.

It's not like leaving the gold standard caused us to borrow based on nothing; we already decided to do that.

Debt is money. Literally - the same contract that's a debt to one party is an asset to the other. And money is just any sufficiently liquid asset.