def _plotting_formula(k, l, m):
    """
    plotting function acc. to DWA-A 531 chap. 5.1.3 for the partial series

    Args:
        k (float): running index
        l (float): sample size
        m (float): measurement period

    Returns:
        float: estimated empirical return period
    """
    return (l + 0.2) * m / ((k - 0.4) * l)
