The ornamental design for the back and seat of a chair, as shown and described.
FIG. 1 is a front perspective view of the back and seat of a chair embodying the new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a left side view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
FIGS. 1-8 show portions of the back and seat of a chair in broken lines. The portions shown in broken lines are for environmental purposes only and form no part of the claimed design. Stipple shading is added to FIG. 1 to convey surface contouring; the stipple shading is not to be understood as conveying a particular surface finish.