Chair base

FIG. 1 is a front and top perspective view of a chair base, showing my new design;

FIG. 2 is a rear and bottom perspective view thereof;

FIG. 3 is a front elevation view thereof;

FIG. 4 is a rear elevation view thereof;

FIG. 5 is a left side elevation view thereof;

FIG. 6 is a right side elevation view thereof;

FIG. 7 is a top plan view thereof;

FIG. 8 is a bottom plan view thereof; and,

FIG. 9 is another perspective view thereof, shown in a using position.

The broken lines in the figures illustrate portions of the chair base that form no part of the claimed design. The broken lines of a chair in FIG. 9 shown in the drawings depict environmental structure that form no part of the claimed design.