blah its a drug topic, so I'm a post machine
looking at the chart more, there are some additional things that need saying.
The "War on Drugs", as opposed to just prohibition, was in main effect in the mid-late 70s, early 80s, not to mention there was a democrat in the whitehouse between 77-81 and 92-...2000ish. This does seem to illustrate that the policy of enforcement has some effect, but conservative enforcement is also associated most with the negative consequences of prohibition.
The period of Clinton's reign also coincides with the adoption of "three-strikes" laws around the states, so it would be hard to categorize those years as being more tolerant of drug use. Also, going by this explanation, one would need to explain how the simple election of GW Bush (who, as far as I know did very little on the drug front) caused the largest drop in LSD use ever recorded.
Further, when looking at other nations, it does appear that draconian penalties for use can curb it, though it took the Taliban boiling users in oil for them to eliminate the marijuana trade.
Just sort of looking at all this, it makes more sense to me that drug use is driven primarily by social forces rather than legal forces. I think dadudemon is the only person I've ever heard of who doesn't use a substance specifically because it is illegal, and it generally holds that for most people that the law is not the most important deciding factor in their choice to use or not.