The one on the Nissan website is really labeled odd to me..."performance level penetration" (y-axis)? What the heck is that?
With the context of the others, it's clearer to me...they're trying to illustrate how 7 deg C is the crossover point where one type of tire is more beneficial than another. If you're significantly above 7 deg C, the summer tire is the most effective, and by the widest margin. As the temperature cools to near 7 deg C, the relative performance difference between the tires mostly vanishes, and none present a compelling argument over another. The further below 7 deg C you go, the less and less effective a summer tire is and the winter tire's advantage, with its softer compound, will continue to grow.
In my personal opinion, I think they should label the y-axis "Relative Performance", and leave it completely unitless. Don't even try to label it with a %. There are too many variables at play. The main tenet is simply that winter tires perform better than summer tires when it's really cold, summer tires perform better than winter tires when it's really hot, and all-season tires play the middle man year-round.
