在ggplot（R）上显示两个平行轴

Question

假设我们有一个简单的以下类型的情节。

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与z完全相关。关系是：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：

如何在x轴中显示变量x和z？它可以是底部的一个，也可以是图表顶部的一个或两个在底部。

Answer 1

这是一次危险的尝试。具有对数比例的先前版本只是错误。

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)

Answer 2

使用出色的 cowplot 库可以实现两全其美的方法。

library(ggplot2)
library(cowplot)

data <- data.frame(temp_c=runif(100, min=-5, max=30), outcome=runif(100))

plot <- ggplot(data) + 
  geom_point(aes(x=temp_c, y=outcome)) + 
  theme_classic() + 
  labs(x='Temperature (Celsius)')

x2plot <- ggplot(data) + 
  geom_point(aes(x=temp_c, y=outcome)) + 
  theme_classic() + 
  scale_x_continuous(label=function(x){round(x*(9/5) + 32)}) + 
  labs(x='Temperature (Fahrenehit)')

x <- get_x_axis(x2plot)
xl <- get_plot_component(x2plot, "xlab-b")

plot_grid(plot, ggdraw(x), ggdraw(xl), align='v', axis='rl', ncol=1, 
          rel_heights=c(0.8, 0.05, 0.05))