Here you'll find a series of graphs, each corresponding to a period of time. The data on each graph represents the average annualized return, per year.
So for example, the data point for 2017 on the 15-year graph represents the average annual return for the past 15 years, as of 2017.
You'll notice that with the exception of individual years, the graphs are all in the positive. This reflects our emphasis on avoiding loss over longer periods: 5 years, 10 years, 15 years or more. Which are the time periods that come to mind when you're planning your retirement.