What is the red "opponent" line on the rating graph?

As the subject says, I can't find an explanation of this line. For some people it seems to wiggle around all over the place, but mine is dead straight. There is not obvious relation between it and my rating.

It's the average rating of your opponents.

as many wonder what is this red line, here is a proposal :

- replace the dots blue and red by a vertical line (as for the training)
- and replace the 2 bubble textes by one indicating both ratings (player's and opponent's average)