R: multiple x axis with annotations

Question

Is it possible to add more than one x-axis to a plot in R? And to put an annotation next to each scale?

Edit > here's the result of Nick Sabbe idea. For the annotation (a little text at the left of each axis), is it possible ?

Answer 1

You can use the line argument of axis() to place an axis higher or lower, this way you can make multiple axes. With mtext() you can then add a label to the side. Do note that the plot itself is only on one scale so you need to rescale the points and labels of the other scale accordingly:

# Plot and first axis:
plot(1:10,1:10,bty="n",col="red",pch=16,axes=FALSE,xlab="",ylab="")
axis(2,0:11,las=1)
axis(1,0:11,line=1,col="red",col.ticks="red",col.axis="red")
mtext("Label 1",1,line=1,at=0.2,col="red")

# Secondary points and axis:
points(rnorm(10,50,20)/10, rnorm(10,5,2),pch=16, col="blue" )
axis(1,0:11,labels=0:11*10,line=3,col="blue",col.ticks="blue",col.axis="blue")
mtext("Label 2",1,line=3,at=0.2,col="blue")

Answer 2

You can use ?axis for that. Parameter at is in the scale of the original axis of the plot, and you can pass labels to show other values.

You have to scale the axess labels yourself, though.

A very simple/silly example:

plot(1:10,1:10)
axis(side=4, at=c(3,7), labels=c(30,70))

Finally, note that most people consider adding multiple axes to a plot bad form...