The ornamental design for a stackableplanter, as shown and described.
FIG. 1 is a front view of a stackable planter embodying our new design.
FIG. 2 is a front elevation view of FIG. 1, the rear view being identical.
FIG. 3 is a side view thereof, the other side view being identical.
FIG. 4 is a top view thereof; and,
FIG. 5 is a bottom view thereof.
The color black in the figures represents a color contrast. The portions of figures shown in broken lines represent environment only and form no part of the claimed design.