def add_custom_options(parser):
    """Adds custom options to a parser.

    :param parser: the parser to which to add options.

    :type parser: argparse.ArgumentParser

    """
    parser.add_argument("--outliers-of", type=str, metavar="POP",
                        default="CEU", choices=["CEU", "YRI", "JPT-CHB"],
                        help=("Finds the outliers of this population. "
                              "[default: %(default)s]"))
    parser.add_argument("--multiplier", type=float, metavar="FLOAT",
                        default=1.9,
                        help=("To find the outliers, we look for more than "
                              "x times the cluster standard deviation. "
                              "[default: %(default).1f]"))
    parser.add_argument("--xaxis", type=str, metavar="COMPONENT", default="C1",
                        choices=["C{}".format(i) for i in xrange(1, 11)],
                        help=("The component to use for the X axis. "
                              "[default: %(default)s]"))
    parser.add_argument("--yaxis", type=str, metavar="COMPONENT", default="C2",
                        choices=["C{}".format(i) for i in xrange(1, 11)],
                        help=("The component to use for the Y axis. "
                              "[default: %(default)s]"))