Respectfully, I disagree.
You could, for instance, mark specific years on a line graph, by using vertical lines — e.g. a vertical line at each of the years mentioned in the article, with an appropriate short label.
The chart in the article doesn’t allow us to compare what’s happening in other years, with the years selected. We’re left with having to take the author’s word for the significance of the data actually selected. That should make the reader wonder why the other data has been left out.
The interval graphed could easily be the same as the interval in the chart. Nothing arbitrary about that. What does seem arbitrary, is selecting data points that make your case (and leaving out ones that weaken it?).
Once again, I’m not trying to argue against the author’s thesis — I’m just pointing out bad graphing practices. There’s a lot of lying with statistics and graphs used against conservatives — it’s something to beware.