如何向森林图添加箭头？

我会在森林图中添加两个箭头，以便将论文发送到期刊。这是来自的演示图metafor::forest:

require(metafor)
data(dat.bcg)
res <- rma(measure="RR", ai=tpos, bi=tneg, ci=cpos, di=cneg, data=dat.bcg,
           slab=paste(author, year, sep=", "))
forest(res, cex=.8, order=order(dat.bcg$ablat), addfit=F)

what I want is to add two arrows along x axis such like(in red box):

有人知道该怎么做吗？

一种想法是使用layout将绘图分为两部分，并用新绘图替换 x 轴标签。

## define the layout matrix  
## 2 rows and 3 columns , the rectangle will be in the cell(2,2)
layout(matrix(c(1,1,1,0,2,0), 2, 3, byrow = TRUE),
       heights=c(3,1),widths=c(1,2,1))
## define the margin since the default ones are usually not enough
par(mar = rep(2, 4))

## your plot here 
data(dat.bcg)
res <- rma(measure="RR", ai=tpos, bi=tneg, ci=cpos, di=cneg, data=dat.bcg,
           slab=paste(author, year, sep=", "))
forest(res, cex=.8, order=order(dat.bcg$ablat), addfit=F,xlab='')

## here all the job 
x <- y <- 2:8 
## dummy plot to define scales
plot(x,y,type='n',axes=F,xlab='',ylab='')
## rectangle
rect(2,4,8,8,border='red')
## arrows
arrows(5.5,6,7,6)
arrows(4.5,6,3,6)
text(6,6,'A better',adj=c(0,1.5),col='blue')
text(3.5,6,'B better',adj=c(0,1.5),col='green')
## x label
text(5,3,'Risk Difference',cex=2)