Add a freeform `p:sp` element having no drawing elements.

        *origin_x* and *origin_y* are specified in slide coordinates, and
        represent the location of the local coordinates origin on the slide.

def _add_freeform_sp(self, origin_x, origin_y):
        """Add a freeform `p:sp` element having no drawing elements.

        *origin_x* and *origin_y* are specified in slide coordinates, and
        represent the location of the local coordinates origin on the slide.
        """
        spTree = self._shapes._spTree
        return spTree.add_freeform_sp(
            origin_x + self._left,
            origin_y + self._top,
            self._width,
            self._height
        )