The ornamental design for a stackingchair, as shown and described.
FIG. 1 is a perspective view of a stacking chair, looking toward the right front corner, showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The broken lines showing of the small screws on the chair leg frames in FIGS. 1, 4 and 6 of the drawing are for illustrative purposes only and form no part of the claimed design.