FIG. 1 is a perspective frontal-side view of a rotor; FIG. 2 is a view from one side thereof; FIG. 3 is a view from the other side thereof; FIG. 4 is a front view thereof; FIG. 5 is a rear view thereof; FIG. 6 is a top view thereof; and, FIG. 7 is a bottom view thereof. The broken lines illustrate the portions of the rotor that form no part of the claimed design.