FIG. 1 is a perspective view of a seat and back unit for a chair;

FIG. 2 is a right side view of a seat and back unit for a chair, the left side being a mirror image thereof;

FIG. 3 is a front view of a seat and back unit for a chair;

FIG. 4 is a rear view of a seat and back unit for a chair;

FIG. 5 is a top view of a seat and back unit for a chair; and,

FIG. 6 is a bottom view of a seat and back unit for a chair.

The broken line showing of a tubular frame in all figures is for environmental purposes only and forms no part of the claimed design.