Sorry, I still disagree with that comment. They're not comparing apples and oranges, here, they're saying "here's why we're using apples to approximate oranges". The graph is showing a close linear relationship between # of cars and stock price.

It's saying "There is some a and b such that:

    a * "number of cars" + b ~ "stock price"

You can have a gripe with that method, sure, but the graph is fine.