The ornamental design for a chair, as shown and described.
FIG. 1 is a perspective view of the top, front and right side of a chair showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear side view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a left view thereof; and,
FIG. 7 is a right view thereof.
The claimed design is indicated by the relatively dark portions in the figures. The portions of the chair shown in faded light grey form no part of the claimed design.