The ornamental design for a chair, as shown and described.
2.1: an top perspective view of the chair.
2.2: a bottom perspective view thereof.
2.3: a front elevational view thereof.
2.4: a rear elevational view thereof.
2.5: a top plan view thereof.
2.6: a bottom plan view thereof.
2.7: a left elevational view thereof.
2.8: a right elevational view thereof.
In the drawings, the dash-dash broken lines show portions of the chair that form no part of the claimed design. The dot-dash-dot lines define the boundaries of the claim and form no part thereof.