The ornamental design for a chair, as shown and described.
FIG. 1 is a perspective view of a chair showing our new design;
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; and,
FIG. 7 is another perspective view of the chair, shown in use with environment.
A bottom view thereof is omitted since it does not contain ornamental features. The broken lines depict environment that forms no part of the claimed design.