I came across that delightful metaphor in an article on how the US economy is in a bubble:
Housing prices are also way overinflated, and another contributor to that household wealth, a big chunk of middle class wealth, he says. Behind it all, is the Fed, which has sprinkled easy-money fairy dust all over the economy. And that central bank is going to bust the household wealth bubble right open, he warns, with rising interest rates and tightening monetary policy.
Analyst who predicted the 2008 crash warns of bubble brewing in U.S. household wealth
So the top blue line is going to “revert to the mean” meaning back to the gold GDP line, which equals a market crash back to reality.