First, I have to disagree with Anton. While the two graphs are different in what they are proving, the measurement of retail square footage in the US does differ, which is what Swamp is referring to. That being said, Swamp is still a moron.
While the two graphs give different data on square footage, which is pointed out by the author, the results still suggest that retail square footage per capita is extremely high in the US, which in turn, strongly supports the whole point of the article, which Swamp of course misses.
As for the sources...the first graph is from a book from the Institute For Local Self Reliance, an organization that has been in existence for 40 years and has done extensive research on the topic. The second one is from a real estate brokerage and consulting firm. I would consider that a source worth of consideration when the topic is retail square footage.
But Swamp, if you have contradictory information on this topic, I'd be happy to look at it.