Create rotated node labels for circle igraph in R

RotateLabels_igraph.R

## How to rotate node labels on circle igraph in R

library(igraph)

### Example
## Generate some fake data
n <- 75
g <- erdos.renyi.game(n, 0.5)
V(g)$name = paste("long_name", 1:n, sep="_")

## laid out as a circle to begin with
la <- layout.circle(g)

par(mar=c(8,6,6,6))
plot(g, layout=la, vertex.size=2, vertex.label="")

## Apply labels manually
#Specify x and y coordinates of labels, adjust outward as desired
x = la[,1]*1.3
y = la[,2]*1.3

#create vector of angles for text based on number of nodes (flipping the orientation of the words half way around so none appear upside down)
angle = ifelse(atan(-(la[,1]/la[,2]))*(180/pi) < 0,  90 + atan(-(la[,1]/la[,2]))*(180/pi), 270 + atan(-la[,1]/la[,2])*(180/pi))

#Apply the text labels with a loop with angle as srt
for (i in 1:length(x)) {
	text(x=x[i], y=y[i], labels=V(g)$name[i], adj=NULL, pos=NULL, cex=.7, col="black", srt=angle[i], xpd=T)
}