This graph allows you to set different tax rates for each industry type. This is useful if you want to promote or retard certain industries. For instance if you think the automobile is about to be the next big thing, try lowering the tax rate for that industry to spur growth. On the other hand, if the pollution from the steel mills is getting you down, increase their tax rate to run them out of town.
You can adjust all the industries at the same time by holding the OPTION key as you drag the bars.