Get keyword args for GoSubDagPlot from self unless they are None.

def get_kws_plt(self):
        """Get keyword args for GoSubDagPlot from self unless they are None."""
        kws_plt = {}
        for key_plt in self.keys_plt:
            key_val = getattr(self, key_plt, None)
            if key_val is not None:
                kws_plt[key_plt] = key_val
            elif key_plt in self.kws:
                kws_plt[key_plt] = self.kws[key_plt]
        return kws_plt