Question

Is there a “automatic” way of adding a second x axis bellow the first one?

My data is similar to this:

A1=data.frame(dates=seq(as.Date("2000/1/1"), by="day", length.out=1080),obs=runif(1080,min=-10,max=15))
A2=data.frame(dates=seq(as.Date("2010/1/1"), by="day", length.out=1080),obs=runif(1080,min=-8,max=50))

par(new=T)
plot(A1,ylim=range(min(A1[,2],A2[,2]):max(A1[,2],A2[,2])))
par(new=F)
plot(A2,ylim=range(min(A1[,2],A2[,2]):max(A1[,2],A2[,2])))

The only solution I found was to make it by manually. I have multiple time series with the same length and I would like to overlay the observations and keep the x axis like this:

_________________________________________________
  |        |       |        |        |        |
2000     2001     2002     2003     2004     2005  
2010     2011     2012     2013     2014     2015
2020     2021     2022     2023     2024     2025
(...)

each time series with a different colour.

Answer 1

Automatically? Not that I know of.

But I do know that you can (with some effort) do this by hand. For example:

par(mar = c(10,4,4,2) + 0.1)
plot(1:3,1:3,axes = FALSE,xlab = "")
axis(1,1:3,labels = FALSE)
mtext(c('2000','2001','2002'),side = 1,line = 1,at = 1:3,col = "red")
mtext(c('2010','2011','2012'),side = 1,line = 2,at = 1:3,col = "blue")
mtext(c('2000','2001','2002'),side = 1,line = 3,at = 1:3,col = "red")
mtext(c('2010','2011','2012'),side = 1,line = 4,at = 1:3,col = "blue")
axis(2)

I’ve increased the number of lines in the bottom margin to 10 by setting mar, to demonstrate how you’d ensure there’s enough room for each row of axis labels.

Then you plot, omitting the axes, add the x axis with just the ticks, and then draw each row of text labels using mtext.

If you didn’t need each row to be a different color, you could do this with one call to axis where the labels argument consists of a vector like c("2000\n2010\n2020","2001\n2011\n2021","2002\n2012\n2022") and then fiddling with padj to get them centered correctly.