Index value plot

This is an index value plot showing the S&P 500 stocks over a ten year time frame. The Values are put up vertically on the y axis, while the x axis shows the number of days during this time period. Although the days on the x-axis are confusing, I still understand this graph and respect its findings.

http://onlamp.com/pub/a/onlamp/2005/11/17/r_for_statistics.html?page=2