def straight_branch(ax, a, b, plot_kws):
    """Draw link line between a and b"""
    a_x, ay = a
    bx, by = b
    branch_x = [a_x, bx, bx]
    branch_y = [ay, ay, by]
    if plot_kws is None:
        plot_kws = {}
    return ax.plot(branch_x, branch_y, **plot_kws)