Plot GO DAGs for each group found under a specfied header GO.

def _get_pltdotstrs(self, hdrgos_usr, **kws):
        """Plot GO DAGs for each group found under a specfied header GO."""
        import datetime
        import timeit
        dotstrs_all = []
        tic = timeit.default_timer()
        # Loop through GO groups. Each group of GOs is formed under a single "header GO"
        hdrgo2usrgos, go2obj = self._get_plt_data(hdrgos_usr)
        # get dot strings with _get_dotstrs_curs
        for hdrgo, usrgos in hdrgo2usrgos.items():
            dotstrs_cur = self._get_dotgraphs(
                hdrgo, usrgos,
                pltargs=PltGroupedGosArgs(self.grprobj, **kws),
                go2parentids=get_go2parents_go2obj(go2obj))
            dotstrs_all.extend(dotstrs_cur)
        sys.stdout.write("\nElapsed HMS: {HMS} to write ".format(
            HMS=str(datetime.timedelta(seconds=(timeit.default_timer()-tic)))))
        sys.stdout.write("{P:5,} GO DAG plots for {H:>5,} GO grouping headers\n".format(
            H=len(hdrgo2usrgos), P=len(dotstrs_all)))
        return sorted(set(dotstrs_all))