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