The ornamental design for a chair, as shown and described.
FIG. 1 is a perspective view of a chair showing my 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.
A bottom view thereof is omitted since it does not contain ornamental features. The broken line portions of the chair in in FIGS. 1-6 represent unclaimed portions of the claimed design and form no part thereof.