The design is embodied in the chair base.
FIG. 1 is an isometric view of the chair base showing the new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a left side elevation view thereof; and,
FIG. 7 is a right side elevation view thereof.
The broken lines shown in the views are for illustrative purposes only and form no part of the claimed design.