The ornamental design for a chair, as shown and described.
FIG. 1 is a front view of a chair showing my new design.
FIG. 2 is a top view thereof;
FIG. 3 is a side view thereof, an opposite side view being a mirror image thereof; and,
FIG. 4 is a rear perspective view thereof.
The circular members presented in broken lines in FIG. 4 are unclaimed features of the chair and form no part of the claimed design. The other broken lines in the figures represent stitching.