You are misinterpreting the graph. It shows the performance of the Top 6 teams relative to each other over the course of the season, and also indicates whether each team is ahead or behind a projected title winning points target. If Chelsea were treading water their line on the graph would be more horizontal, instead it is diagonal which indicates a constant decline which started around the 14th game. This decline is relative to the other Top 6 teams as well as the baseline. I'm sorry if you are unable interpret the graph correctly, but it does do a good job of tracking the performance of the top teams using two important metrics IMO.