The ornamental design for a chair, as shown and described.
FIG. 1 is a perspective view of a chair;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a left side view thereof; and,
FIG. 7 is a right side view thereof.
The broken line showing fasteners and slits in FIG. 1 and FIG. 5 is included for the purpose of illustrating environmental structure and forms no part of the claimed design.