NYSE Composite Index - 1971

The following graph plots the year-to-date performance of the index for 1971 compared to the probability calculated by a random walk model.

Initial Value: 531.12 Hist. Volatility: 0.74% Annual Drift: 7% Final Value: 596.68 Percentile: 66%
Click on the image for the high resolution version.

The high to date and low to date prices of the stock plotted against a random walk model for 1971 (what's this?).