在 ggplot 上显示两个平行轴 (R)

假设我们有以下类型的简单情节。

library(ggplot2)
df = data.frame(y=c(0,1.1,2.3,3.1,2.9,5.8,6,7.4,8.2,9.1),x=seq(1,100, length.out=10))
ggplot(df,aes(x=x,y=y)) + geom_point()

x完全相关z。其关系为：Constant=x^2*z=1.23因此我可以像这样重写 data.frame ：

df = cbind(df,1.23/df$x^2)

问题是：

如何显示这两个变量xand zx 轴之一？它可以是一个在图表的底部，一个在图表的顶部，或者两者都在底部。

这是一个危险的尝试。以前带有对数刻度的版本只是wrong.

library(ggplot2)
df = data.frame(y=c(0,1.1,2.3,3.1,2.9,5.8,6,7.4,8.2,9.1),
                x=seq(1,100, length.out=10))
df$z = 1.23/df$x^2


## let's at least remove the gridlines
p1 <- ggplot(df,aes(x=x,y=y)) + geom_point() +
  scale_x_continuous(expand=c(0,0)) +
  theme(panel.grid.major=element_blank(),
        panel.grid.minor = element_blank())

## make sure both plots have expand = c(0,0) 
## otherwise data and top-axis won't necessarily be aligned...
p2 <- ggplot(df,aes(x=z,y=y)) + geom_point() +
  scale_x_continuous(expand=c(0,0))

library(gtable)
g1 <- ggplotGrob(p1)
g2 <- ggplotGrob(p2)
tmp <- gtable_filter(g2, pattern="axis-b")

## ugly tricks to extract and reshape the axis
axis <- tmp[["grobs"]][[1]][["children"]][["axis"]] # corrupt the children
axis$layout <- axis$layout[2:1,]
axis$grobs[[1]][["y"]] <- axis$grobs[[1]][["y"]] - unit(1,"npc") + unit(0.15,"cm")
## back to "normality"    

g1 <- gtable_add_rows(g1, sum(tmp$heights), 2)
gtableAddGrobs <- gtable_add_grob # alias, making sure @!hadley doesn't see this
g1 <- gtableAddGrobs(g1, 
                     grobs=list(gtable_filter(g2, pattern="xlab"),axis), 
                     t=c(1,3), l=4)
grid.newpage()
grid.draw(g1)