plot with only horizontal gridlines and labels - r

Question

I have some barplot
barplot(1:3, axes=FALSE)
to which I would like to add horizontal gridlines with labels using base R.
I get the first part using
par(xpd = TRUE, mai=c(0.5,1,0.5,0.2)) # to extend lines to the left of plotting area
barplot(1:3, axes=FALSE) # plotting area
grid(nx=0, ny=3, col="gray") # horizontal grid lines
barplot(1:3, axes=FALSE, add=TRUE) # get grid lines into background
but I fail to get the labels on the left end. That is, at the left end of the gridlines, I would like to have the Y-values, here 1 and 2.

Answer 1

The documentation of ?grid gives a solution to this problem. From section Note.
Note
If more fine tuning is required, use abline(h = ., v = .) directly.
old_par <- par(xpd = TRUE, mai=c(0.5,1,0.5,0.2))
barplot(1:3, axes = FALSE)
abline(h = 1:3, col = "grey", lty = "dotted")
barplot(1:3, axes = FALSE, add = TRUE)
text(x = -0.5, y = 1:3, labels = 1:3)
par(old_par)
To have the y axis labels at the end of the grid lines and to place them automatically, a function can be defined.
segmText <- function(x0, x1, y, ...){
segments(x0 = x0, x1 = x1,
y0 = y, y1 = y, ...)
text(x = x0, y = y, labels = y)
}
old_par <- par(xpd = TRUE, mai = c(0.5,1,0.5,0.2)) # to extend lines to the left of plotting area
barplot(1:3, axes = FALSE)
segmText(x0 = -0.5, x1 = 4, y = 1:3, col = "grey", lty = "dotted")
barplot(1:3, axes = FALSE, add = TRUE)
par(old_par)
Edit
A simpler solution seems to be in the comment by user d.b
graphics.off()
barplot(1:3, axes = FALSE, col = NA, border = NA)
abline(h = 1:3, col = "grey", lty = "dotted")
barplot(1:3, axes = FALSE, add = TRUE)
axis(2, at = 1:3, labels = 1:3, las = 2, col = NA)

Answer 2

You could use lines() within a sapply() and use mtext(). Plot an empty plot beforehand.
plot(x=0:4, y=0:4, type="n", axes=FALSE, xlab="", ylab="")
sapply(1:3, function(x) lines(0:4, rep(x, 5), lty=3, col="gray"))
barplot(1:3, axes=FALSE, add=TRUE)
mtext(1:3, side=2, line=0, at=1:3, las=1, adj=0)
You can trim this by changing e.g. line=-1 or play around with adj.
Produces