def m(x0: float, x1: float, y0: float, y1: float) -> float:
    """
    Simple gradient function.

    Parameters
    ----------
    x0 : float
        x co-ordinate at time 0.

    x1 : float
        x co-ordinate at time 1.

    y0 : float
        y co-ordinate at time 0.

    y1 : float
        y co-ordinate at time 1.

    Returns
    -------
    grad : float
        Gradient value.

    Notes
    -----

    """
    grad = (y1 - y0) / (x1 - x0)

    return grad