I claim the ornamental design for a backrest for a chair, as shown and described.
FIG. 1 is an upper perspective view of the backrest for a chair;
FIG. 2 is a lower perspective view of the design shown in FIG. 1;
FIG. 3 is a front plan view of the design shown in FIG. 1;
FIG. 4 is a back plan view of the design shown in FIG. 1;
FIG. 5 is a left side plan view of the design shown in FIG. 1;
FIG. 6 is a right side plan view of the design shown in FIG. 1;
FIG. 7 is a top plan view of the design shown in FIG. 1; and,
FIG. 8 is a bottom plan view of the design shown in FIG. 1.
The broken lines shown in the drawings are for the purpose of illustrating environmental subject matter and form no part of the claimed design. The shade lines shown represent surface shading and not surface ornamentation, color, or material type.