Why would the start point make a big difference? Is the assertion that if it started at 1820, you’d see a stair step and then flat from 1920 on? Either way, th

Why would the start point make a big difference? Is the assertion that if it started at 1820, you’d see a stair step and then flat from 1920 on? Either way, these graphs look flat from 1920 on. That’s the point of the article.