True. Bond yields would be super useful. In 1961, 10yr US treasury yields were 3.84%, and in 1971 they were 6.24%, so you'd have made some 5% nominally by rolling (I don't have 20yr yields at hand), or -0.7% in real terms, beating equity handily.

So, for that period, treasuries > shares > cash.

HOWEVER, I think the chart is not so useful (or intended) as a decision aid for asset allocation (equity/debt/...), but quite good to disabuse people of that notion that "over the long term, you can't lose with equities", or the idea that you can pretty much rely on a 6% real return on your savings.