The ornamental design for a chair, as shown and described.
FIG. 1 is a perspective view of the chair;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken line portions of the drawings are included to show portions of the article that form no part of the claimed design and represent environmental structure only.