The ornamental design for a grabbar having a decorative surface, as shown and described.
FIG. 1 is a front perspective view of a grab bar having a decorative surface showing my new design;
FIG. 2 is a top view of a grab bar having a decorative surface;
FIG. 3 is a front view of a grab bar having a decorative surface;
FIG. 4 is a right side view of a grab bar having a decorative surface; and,
FIG. 5 is a bottom view of a grab bar having a decorative surface.
Left side views not shown are mirror images of right side views. Any broken lines illustrative of environmental structure in the drawings are not part of the design sought to be patented.