The graphic isn't presented to compare stocks with the alternatives of cash/bond yields. I think the major service done here is setting a realistic expectation for the returns you might see in your lifetime. It's especially relevant for people who's major investing periods have/will occur in the 2000-2020 timeframe of sluggish growth and ultra-low yield. The Jeremy Siegel '8%' number is a really dangerous number to set your expectations on when saving. If you do, you might be 20 years in and well outside the bounds of a timeframe where compound interest can help you live the retirement you were aiming for before you realize your mistake.
On scale, it makes it seem like +3% to +7% real returns is "neutral". This makes it seem like the stock market is sometimes good sometimes bad but overall it may as well be just okay.
On comparisons, it does a huge disservice by not adding a tab showing bond yields and a tab showing cash/treasury yields (which would be dark red across the board except light red around 1930).
I feel these slights make the graphic present stock investing in an unfairly unfavorable light and makes the suboptimal strategy of keeping your money out of the market seem much more favorable than it is.