The ornamental design for a chair, as shown and described.
FIG. 1 is a front perspective view of a chair;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof; and,
FIG. 6 is a top view thereof.
The broken lines in the drawings depict unclaimed subject matter and form no part of the claimed design. The bottom view is not seen during normal use. Therefore, it is omitted.