R: Positioning graph elements using cm units with generic X-Y Plotting functions

Question

I am sure this is not new for the R community, but is new to me and can't find a clear answer. Assuming this example:

plot(1:10, xlab="", xaxt="n") # supress OX axis
title(xlab="How can I use cm?", line=2.5)
axis(side=1, at=1:10, line=0.2) 

Here I used line argument in function title() to place a label at 2,5 lines of text "outwards from the plot edge" (as described in ?title help). Is there any argument that can take cm, or a way to use cm? Also, how can I find out how many cm does a line of text contains (if there is no other way around)?

Would also be great to know/set the margins in cm and not only like par("mar") [lines of text] or par("mai") [inches]. Is there a way to do that?

Answer 1

Using the line2user function from this answer you can convert centimeters to a "line" then convert the line to user coordinates and add things to the plot using xpd = TRUE :

cm2line <- function(x) {
  lh <- par('cin')[2] * par('cex') * par('lheight')
  inch <- x/2.54
  inch/lh
}

par(mai = rep(5/2.54, 4))
plot.new() 
box()
mtext("hello", side = 3, line = cm2line(2))
abline(h = line2user(cm2line(1:5), side = 4), xpd = TRUE)
abline(h = line2user(cm2line(1:5), side = 1), xpd = TRUE)
abline(v = line2user(cm2line(1:5), side = 2), xpd = TRUE)
abline(v = line2user(cm2line(1:5), side = 3), xpd = TRUE)