I believe gravypod is referring to previous years in the graph that starts the article. For instance, look at the 2008-2012 period. There is a definite downward trend.[0]

[0]: https://img.washingtonpost.com/blogs/wonkblog/files/2017/10/...

Yes, exactly. To prove a correlation, in my opinion, one would need....

    1. More smoothed/averaged dataset (centered rolling average ~4 months?)
    2. Explain the other extreme dips in this graph (2005, 2007, 2011)
    3. Plot similar data from other states (with and without legal marijuana)
    4. Investigate other causes (see sparrish's comment)

Is that a joke? The source is the linked article. The chart hardly shows a convincing downturn. The trend lines are very generously positioned.

This is opinion, presumably informed by the graphic at the very top of the linked article. Asking for a source for an opinion is a little silly.