Does reality drive straight lines on graphs, or do straight lines on graphs drive reality?

Here’s a graph of US air pollution over time:

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. If we wanted to be really harsh, we could make a graph like this:

But the same argument that disproves the importance of photolithography disproves the importance of anything else. We’d have to retreat to a 1000-coin-flips model where each factor is so small that it happening or not happening at any given time doesn’t change the graph in a visible way.

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