def _powerlaw(x0, y0, slope, x):
    """
    From point (x0, y0) and parameter slope, returns y = f(x) such that:
    > f(x) = a * (x ** slope)
    > f(x0) = y0

    Parameters
    ----------
    x0, y0, slope, x : float

    Returns
    -------
    y = f(x) : float
    """

    return y0 * ((x/x0) ** slope)