That chart is a bit misleading when you account for the volume coming from High Frequency Trading.
From January 27, 2010 (so it's recent):
http://www.tradersmagazine.com/news/high-frequency-trading-stocks-volume-104994-1.html
Take out 55% of the volume, and what would that chart look like?
Yes, the article quotes Nasdaq numbers, but DJIA numbers are about the same.