That's a damn good question and I have been wondering about that myself. I have been trying to compare many of the averages and even a few stocks to the $USD (as a ratio like this)
It is interesting sometimes, but I haven't gotten a whole lot of useful data from it. If you look at that chart on a weekly basis, the ratio is about as high as it has ever been.
The last time the markets got out of hand vs. the $USD was probably 1987. That lead to the crash, but these charts don't provide much useful info to compare to that time period.
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