Why slip is negative in this plot?
Positive slip is defined as how much rotor lags behind the rotating magnetic field when it is rotoring, due to the load torque resisting the torque applied by the rotor.
And if zero slip is defined as when there is no-load and the rotor is aligned with the rotating magnetic field, then it only stands to reason that negative slip is how much the rotor leads the rotating magnetic field when it is generating, due to the load torque being applied to the rotor in a way that assists the machine rather than resisting it.