The ornamental design for a seat of a chair, as shown and described.
FIG. 1 is a side elevation view of the seat of a chair showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a partial view, shown at a larger scale, of a surface structuring of an upper surface of the seat of a chair;
FIG. 5 is a front, top perspective view of the seat of a chair;
FIG. 6 is a rear elevation view thereof; and,
FIG. 7 is a rear and side perspective view thereof.
The dash dot broken lines shown in FIG. 4 are for the purpose of illustrating boundaries of the claimed design in an enlarged partial view and form no part thereof. The dash-dash broken lines shown in FIG. 7 are for the purpose of illustrating portions of the article and form no part of the claimed design.