如何绘制镜像图？ （两个图共享相同的x轴，但一个图上下颠倒-在该轴下方）

Question

Suppose I have two samples that I would like to compare graphically. One way to do it is to lay one over another like below:

x1 = rnorm(100)
x2 = rnorm(100, mean=2)
plot(density(x1))
lines(density(x2), col="red")

I wonder however if there is a way to plot x2 such that the plot shares the same axis as the plot of x1, except that it is upside down, like the plot below. It would be especially great if there is any method that does not involve downloading additional packages.

Thanks!

Answer 1

If it doesn't matter if the y-axis contains values below zero, you can use this:

x1 <- rnorm(100)
x2 <- rnorm(100, mean=2)
dens1 <- density(x1)
dens2 <- density(x2)
dens2$y <- dens2$y * -1
plot(dens1, 
     ylim = range(c(dens1$y, dens2$y)),
     xlim = range(c(dens1$x, dens2$x)),
     main = "",
     xlab = "")
lines(dens2, col = "red")

Answer 2

You can reverse the axis of a plot by using the argument ylim=(...) (or xlim=(...) ) and specifying the limits in the reverse order.

For example:

layout(matrix(1:2, ncol=1))
par(mai=c(0.5, 1, 0.5, 1))

plot(c(-6, 6), 0:1, type="n", ylim=c(0, 1), xlab="", ylab="")
lines(density(x1), ylim=c(0, 1))

plot(c(-6, 6), 0:1, type="n", ylim=c(1, 0), xlab="", ylab="")
lines(density(x2), col="red", ylim=c(1, 0))

Answer 3

It is possible to get only one x-axis using the mar argument of the par function. The R code would look like this:

   #Create Data
    x1 = rnorm(100)
    x2 = rnorm(100, mean=2)

    #Make the plot
    par(mfrow=c(2,1))
    par(mar=c(0,5,3,3))
    plot(density(x1) , main="" , xlab="", ylim=c(0,1) , xaxt="n", las=1 , col="slateblue1" , lwd=4 )
    par(mar=c(5,5,0,3))
    plot(density(x2) , main="" , xlab="Value of my variable", ylim=c(1,0) , las=1 , col="tomato3" , lwd=4)

Giving this plot:

1

This graph is present in the R graph gallery