Swivel Chair Base

FIG. 1 is a back, left, top perspective view of a swivel chair base showing my new design;

FIG. 2 is a front view thereof;

FIG. 3 is a back view thereof;

FIG. 4 is a left view thereof;

FIG. 5 is a right view thereof;

FIG. 6 is a top view thereof; and,

FIG. 7 is a bottom view thereof.

The broken lines shown on the drawings depict portions of the swivel chair base that form no part of the claimed design.