我的程序如下， 运行后得到下图，，我想在两条垂直虚线与横坐标轴相交处分别标记字母a，b，但是不显示对应的刻度，怎么办呢，多谢各位了！

h=function(b){
w=(1+b^2)/(1+(b-1)^2)
2*log(w)
}
x=seq(from=-5,to=5,by=0.01)
y=h(x)
plot(x,y,type='l')
arrows(-0.25,-1.76, -5, -1.76, length = 0,lty = 2)
arrows(1.25,1.76, 5, 1.76, length = 0,lty = 2)
arrows(-0.25,-2.1, -0.25, 2, length = 0,lty = 3)
arrows(1.25, -2.1, 1.25, 2, length = 0,lty = 3)