CLAIMThe ornamental design for a combined seat and backrest unit for a chair, substantially as shown and described.DESCRIPTIONFIG. 1 is a front perspective view from above of the chair embodying our new design;FIG. 2 is a rear perspective view from below of the chair;FIG. 3 is a front view of the chair;FIG. 4 is a rear view of the chair;FIG. 5 is a plan view of the chair; andFIG. 6 is a side view of the chair. The broken lines disclosing a chair base in FIGS. 1-3 are shown for illustrative purposes only.