def rotate_axes(xs, ys, zs, zdir):
    """
    Reorder coordinates so that the axes are rotated with zdir along
    the original z axis. Prepending the axis with a '-' does the
    inverse transform, so zdir can be x, -x, y, -y, z or -z
    """
    if zdir == 'x':
        return ys, zs, xs
    elif zdir == '-x':
        return zs, xs, ys
    elif zdir == 'y':
        return zs, xs, ys
    elif zdir == '-y':
        return ys, zs, xs
    else:
        return xs, ys, zs
