The ornamental Design for a chair, as shown and described.
FIG. 1 is a perspective view from the rear and to one side of the chair; and,
FIG. 2 is a rear elevational view of the chair of FIG. 1.
The base, control mechanism, arms, seat and the portion of the chair back shown in dotted outline do not form part of the design. The boundary of the design is distinguished by dot-dash lines in each Figure.