Each black data point in the graph represents a month in the stock market and is the average of the daily closing prices of the S&P 500 for that month adjusted for inflation by the consumer price index with dividends reinvested. The data before the existence of the S&P 500 was calculated by Cowles and Associates using the methodology of the S&P 500. In 1870 there were only 10 stocks in the index.
The green line shows the best fit growth rate in purchasing power in the period since the modern inflation trend began. The red line shows the best fit for the period prior to that.
The shaded area shows periods where the stock market with reinvested dividends lost value to inflation.