def _add_subparser(cls, subparsers, parent):
        r"""
        Adds a specific action based sub-parser to the supplied arg_parser
        instance.
        """
        parser = subparsers.add_parser(cls.__name__,
                                       aliases=['histlog'],
                                       parents=[parent],
                                       help=cls.__doc__)
        #
        parser.add_argument('scale_fact', type=float, nargs='?', default=10.0,
                            help='base to generate logscale from')
        parser.set_defaults(func=cls)