The ornamental design for a chair, as shown and described.
FIG. 1 is a front perspective view of an embodiment of my design;
FIG. 2 is a back perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a back view thereof;
FIG. 5 is a right side view thereof, the left side being a mirror image thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines in the figures illustrate portions of the chair that form no part of the claimed design. All other surface(s) or portion(s) of my chair not shown in the figures or otherwise described in the specification form no part of the claimed design.