The ornamental design for a chair, as shown and described.
FIG. 1 is a front perspective view of a chair;
FIG. 2 is a front view of a chair;
FIG. 3 is a back view of a chair;
FIG. 4 is a right side view of a chair;
FIG. 5 is a left side view of a chair;
FIG. 6 is a top view of a chair; and,
FIG. 7 is a bottom view of a chair.
The broken lines showing portions of the design in all figures is included for the purpose of illustrating environment and unclaimed features and forms not part of the claimed design.