The ornamental design for a chair, as shown and described.
FIG. 1 is a front elevational view of the chair comprising my new design;
FIG. 2 is a right side elevational view thereof, the opposite side elevational view being a mirror image thereof; and,
FIG. 3 is a top plan view thereof.
The broken lines showing a seat cushion and the central portion of the backrest are for environmental purposes only and form no part of the claimed design.