This chart gets the P/E from Case-Shiller U.S. Real Home Price Index P/E, P, E - Monthly and adds horizontal lines. The horizontal gray line is the average index value. The colored horizontal lines represent one and two standard deviations from the average. By definition, around 68% of the data points are within one standard deviation of the average and 95% of the data points are within two standard deviations of the average.
mccormick-charts.com: Case-Shiller U.S. Real Home Price Index P/E, P, E - Monthly
external: Case Shiller Index, standard deviation