Backrest for Chair

FIG. 1 is a perspective view of my new design, showing a backrest for chair; FIG. 2 is another perspective view thereof; FIG. 3 is a front elevation view thereof; FIG. 4 is a rear elevation view thereof; FIG. 5 is a left side elevation view thereof; FIG. 6 is a right side elevation view thereof; FIG. 7 is a top plan view thereof; and, FIG. 8 is a bottom plan view thereof. The line shading in FIGS. 1 - 8 represents contour and not surface ornamentation. The broken line showing of portions of the backrest for chair is included for the purpose of illustrating environmental structure and forms no part of the claimed design.