It’s just s&p 500 index value divided by gold to get a “gold native” number. The compounding represented here would be the growth of the companies that has been “compounded” as a result of their internal investment
It’s ignoring S&P dividends which add a significant amount.
S&P on its own is up 20 fold since 1990
SP500TR Is up 40 fold.
I guess if your purpose of the site is to make yourself feel good about gold they chose the proper index. Agreed if you want to compare the decidedly non-dividending gold as an asset class TR makes sense.
Yes, these charts keep popping up on my Twitter and it's always difficult to know what they are really measuring.