The success of employee drug testing

Via CESAR Fax (may not yet be up on their site), comes this chart showing a 10 year comparison of employee positive drug tests from Quest Diagnostics. The Quest statistics are often trotted out in cherry-picked portions to proclaim a dramatic reduction in cocaine use or marijuana use or meth use, but rarely showing the whole picture.
Is this supposed to be good?

Overall, the total percentage of workers with positive drug tests showed an insignificant reduction, one that could be more than offset by the substitution effect (switching from marijuana that stays in your blood a long time to other drugs that don’t).
I don’t see how anyone can call this chart an example of success.
Also, notice the workplace drug that’s missing?