The bi-square weight function calculated over values of xx

    Parameters
    ----------
    xx: float array

    Notes
    -----
    This is the first equation on page 831 of [Cleveland79].

def bi_square(xx, idx=None):
    """
    The bi-square weight function calculated over values of xx

    Parameters
    ----------
    xx: float array

    Notes
    -----
    This is the first equation on page 831 of [Cleveland79].
    """

    ans = np.zeros(xx.shape)
    ans[idx] = (1-xx[idx]**2)**2
    return ans