The ornamental design for a chair, as shown and described.
FIG. 1 is a front-left perspective view of a first embodiment of a chair showing our new design;
FIG. 2 is a front perspective view thereof;
FIG. 3 is a left side perspective view thereof;
FIG. 4 is a top perspective view thereof;
FIG. 5 is a front-right perspective view of a second embodiment of a chair showing our new design, the second embodiment being a mirror image of the first embodiment;
FIG. 6 is a front perspective view thereof;
FIG. 7 is a right side perspective view thereof; and,
FIG. 8 is a top view perspective thereof.
The unshown rear sides and underside of the chair form no part of the claimed design.