Plot an exploratory GO DAG for a single Group of user GOs.

def _plot_go_group(self, hdrgo, usrgos, pltargs, go2parentids):
        """Plot an exploratory GO DAG for a single Group of user GOs."""
        gosubdagplotnts = self._get_gosubdagplotnts(hdrgo, usrgos, pltargs, go2parentids)
        # Create pngs and return png names
        pngs = [obj.wrplt(pltargs.fout_dir, pltargs.plt_ext) for obj in gosubdagplotnts]
        return pngs