I've never seen market valuation expressed as market cap as % of GDP. I'm not an economist, so I'll leave the detailed arguments to them. But it would be at least useful to explain why you think this is a meaningful metric as compared to those typically used to measure market valuation (e.g. P/E ratios etc.).
Your graph also ties your valuation metric to the 2000 peak and the 2008 peak. However, there were crashes in 1990 and 1987 as well. Should readers conclude that the 1987 peak level was also too high, and that therefore the last ~30 years have also been too high as well? (Abstaining from investing in the stock market at levels above the 1987 crash would have resulted in the loss of tremendous opportunity for wealth creation.)
There are a lot of opinions implicitly expressed in this site; it would be good to try to make those explicit.
Your graph also ties your valuation metric to the 2000 peak and the 2008 peak. However, there were crashes in 1990 and 1987 as well. Should readers conclude that the 1987 peak level was also too high, and that therefore the last ~30 years have also been too high as well? (Abstaining from investing in the stock market at levels above the 1987 crash would have resulted in the loss of tremendous opportunity for wealth creation.)
There are a lot of opinions implicitly expressed in this site; it would be good to try to make those explicit.