def _get_point_on_line(x1, y1, x2, y2, n):
    """Returns the (x, y) tuple of the point that has progressed a proportion
    n along the line defined by the two x, y coordinates.
    """
    x = (x2 - x1) * n + x1
    y = (y2 - y1) * n + y1
    return x, y
