I claim the ornamental design for a chair, as shown and described.
FIG. 1 is a front perspective view of a chair showing my new design;
FIG. 2 is a right side view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The broken lines are for the purpose of illustrating environment and form no part of the claimed design. Any surfaces of the design not shown, including interior surfaces, form no part of the claimed invention.