The ornamental design for an armrest for a chair, as shown and described.
FIG. 1 is a perspective view showing the front, left side, and top of two armrests for a chair showing my new design, the broken lines of a chair being for illustrative purposes only and forming no part of my design;
FIG. 2 is an enlarged perspective view of one such armrest for a chair;
FIG. 3 is a top plan view thereof;
FIG. 4 is a left side elevational view, the right side elevational view being a mirror image thereof; and,
FIG. 5 is a front elevational view thereof.
The characteristic feature of the design resides in the application of a groove pattern resembling the tread on an automobile tire to the armrest for a chair.