The ornamental design for a chair, as shown and described.
FIG. 1 is a top, front, right perspective view of a chair showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof; and,
FIG. 6 is a top view thereof.
The broken lines immediately adjacent to the shaded area define the bounds of the claimed design and form no part thereof. The broken lines depicting the remainder of the chair form no part of the claimed design.