The ornamental design for a stackingpin, as shown and described.
FIG. 1 is an isometric view of the front of the stacking pin, showing my new design;
FIG. 2 is an enlarged top plan view of the stacking pin thereof, the bottom plan view being a mirror image; and,
FIG. 3 is a front elevational view of the stacking pin thereof, the back elevational view, right side elevational view, and left side elevational view being mirror images.