The ornamental design for a backrest for swing or bench, as shown and described.
FIG. 1 is a perspective view of a backrest for swing or bench showing my new design.
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a side view thereof;
FIG. 5 is an opposite side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The features presented in broken lines represent unclaimed portions of the backrest for swing or bench and unclaimed environmental structure and form no part of the claimed design.