Does Reality Drive Straight Lines on Graphs, or Do Straight Lines on Graphs Drive Reality?

Summary: Here’s a graph of US air pollution over time: During the discussion of 90s environmentalism, some people pointed out that this showed the Clean Air Act didn’t matter. The trend is the same before the Act as after it. This kind of argument is common. For example, here’s the libertarian Mercatus Institute arguing that OSHA didn’t help workplace safety: I’ve always taken these arguments pretty seriously. But recently I’ve gotten more cautious.