The ornamental design for a chair back, as shown and described.
FIG. 1 is a perspective view showing the front and left side of a chair back which forms my new design, the alternating long and short broken lines showing a chair seat being for illustrative purposes only and forming no part of my design;
FIG. 2 is a front elevational view thereof; and,
FIG. 3 is a left side elevational view thereof, this view and the opposite right side elevational view being mirror images.
The short broken lines shown in FIGS. 1-3 are understood to represent stitching.