The ornamental design for a chair, as shown and described.
FIG. 1 is a perspective view of a chair of my new design (the shading is not intended to indicate anything in particular);
FIG. 2 is a front view of thereof;
FIG. 3 is a left-side view thereof, the right-side view being a mirror-image; and,
FIG. 4 is a top view thereof.
The broken lines in drawings, indicating that no particular cushion is being claimed, represent environmental subject matter and form no part of the claimed design.