The ornamental design for chair, as shown and described.
1. Chair
1.1 : Front
1.2 : Back
1.3 : Right
1.4 : Top
1.5 : Bottom
1.6 : Cross-sectional
1.7 : Right view with seat raised
1.8 : Right view of stacked chairs
The portions of the reproductions shown in dash-dash broken lines are not part of the claimed design. The dot-dash-dot broken lines represent the boundaries of the claim and form no part thereof. The left side view is a mirror image of the right side view of 1.3 and therefore not shown. 1.6 is a cross-sectional view, vertically sectioned at the center of the chair.