The ornamental design for a chair back, as shown and described.
FIG. 1 is a front elevational view of a chair back, showing my new design,
the broken lines are for environmental purposes only and form no part of
the claimed design;
FIG. 2 is a rear elevational view thereof; and,
FIG. 3 is a right side view thereof, a view taken from the left side being
a mirror image thereof.