The ornamental design for a stackableshelf, as shown and described.
FIG. 1 is a perspective view of a stackable shelf showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof;
FIG. 8 is another perspective view thereof with drawers of the stackable shelf pulled out of the stackable shelf; and,
FIG. 9 is the other perspective view thereof with an additional stackable shelf drawn in broken lines.
The broken line portions of the stackable shelf in FIGS. 1-9 represent unclaimed portions of the claimed design and form no part thereof. The broken lines of the additional stackable shelf in FIG. 9 illustrate unclaimed environment and forms no part of the claimed design.