The ornamental design for a screen for a chair, as shown and described.
FIG. 1 is a front left top perspective view of a screen for a chair, showing my new design, the rear left top perspective view being a mirror image thereof with the exception of a disclaimed hanging loop;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof, the right side elevation view being a mirror image thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is bottom plan view thereof.
The broken lines shown in the drawings depict a hanging loop and coupling regions that form no part of the claimed design.