The ornamental design for a skateboard with a movable circular center, as shown and described.
FIG. 1 is a perspective view of a skateboard with a movable circular center the novelty of the design resides in the shape and configuration of the article as shown, showing my new design;
FIG. 2 is a front elevational view of the invention; the rear elevational view being mirror image thereof;
FIG. 3 is a left-side elevational view of the invention, the right-side elevational view being a mirror image thereof;