The ornamental design for a chair, as shown and described.
FIG. 1 is a perspective view of a chair.
FIG. 2 is a front elevation view thereof.
FIG. 3 is a rear elevation view thereof.
FIG. 4 is a right side elevation view thereof.
FIG. 5 is a left side elevation view thereof.
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The portions of the chair shown in broken lines are for the purpose of showing environmental structure only and form no part of the claimed design. The backrest sheet has a mesh pattern and this backrest sheet forms no part of the claimed design.