How to interpret derating charts for resistors
The chart is showing you how much power the resistor can safely dissipate as a function of ambient temperature.  At 70 &deg;C or below is can dissipate full rated power.  Above that, its max allowable dissipation goes down until it can't dissipate anything anymore at about 270 &deg;C.

The solid versus dotted line probably means that other specs are violated above 150 &deg;C ambient, but they show you the full power dissipation relationship anyway, all the way down to can't dissipate anything more without getting cooked.