Display two parallel axes on a ggplot (R)

Question

Let's say we have a simple plot of the following kind.

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 perfectly correlates with z. The relation is: Constant=x^2*z=1.23 therefore I could rewrite the data.frame like this:

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

The question is:

How can I display both variables xand zone the x-axis? It could be one at the bottom and one at the top of the graph or both at the bottom.

it's not possible in ggplot2. It could be hacked with grid, but every time one does that, hadley kills a Camel. — baptiste, Aug 29 '13 at 11:34
If you want secondary axes don't use `ggplot2`. It's much easier with base graphics (or possibly lattice). — Roland, Aug 29 '13 at 12:18
If anybody comes here after such a long time: this is now supported by ggplot2 http://ggplot2.tidyverse.org/reference/sec_axis.html The doc only shows a secondary y example, but it works as well with `scale_x_continuous` — xav, Mar 26 '18 at 10:47

baptiste · Accepted Answer · 2016-04-16T07:18:13.167

Here's a dangerous attempt. Previous version with a log-scale was just 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)

enter image description here

score 0 · Answer 2 · answered Jan 06 '21 at 00:42

A both-on-the-bottom approach can be done with the excellent cowplot library.

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))

Display two parallel axes on a ggplot (R)

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