def __adjust_extent(xmin, xmax, ymin, ymax):
    """
    Ajust the extent of the plot extent.
    
    Parameters
    ----------
    xmin: float
        minimum longitude.
    xmax: float
        maximum longitude.
    ymin: float
        minimum latitude.
    ymax: float
        maximum latitude.
    
    Returns
    -------
    re: tuple
        tuple of adjusted (xmin, xmax, ymin, ymax).
    """
    if xmax - xmin < 15:
        xmin -= 15
        xmax += 15
    elif xmax - xmin < 25:
        xmin -= 10
        xmax += 10
    else:
        xmin -= 5
        xmax += 5
    
    if ymax - ymin < 12:
        ymin -= 12
        ymax += 12
    elif ymax - ymin < 18:
        ymin -= 8
        ymax += 8
    else:
        ymin -= 4
        ymax += 4
    
    return xmin, xmax, ymin, ymax