The author didn't say anything about listing trends on those other sites, only that their listings usually trend closely with the others, using the past six mon

The author didn't say anything about listing trends on those other sites, only that their listings usually trend closely with the others, using the past six months as evidence. He shows us the first six months of each year, with only his site. The only explanation I can come up with is that the absent data doesn't jive well enough with the thesis. One chart could have made all this more clear: January 2014 through June 2016, with all the sites mentioned included in the chart. The author hints at having that data available, but has left out big chunks of it. I can understand not wanting to dump so much business intelligence on some website, but they're the ones trying to write about the data without showing it.