Add legend in igraph to annotate difference vertices size

Question

I have a graph in igraph with a vertex size that is coded for a value.
I wish to add a legend with symbols (points in my case) with different sizes. I tried the following code

require(igraph)
er_graph <- erdos.renyi.game(100, 5/100)
value<-runif(100)
n<-6
size_vec<-seq_len(n)
sizeCut<-cut(value,n)
vertex.size<-size_vec[sizeCut]
plot(er_graph, vertex.label=NA, vertex.size=vertex.size)
legend('topleft',legend=levels(sizeCut),pt.cex=size_vec,col='black')

but end with legend without symbols see example

Any sugestions how I go about this?

Answer 1

You should set pch to some value to indicate which character you want to use for the bullets (see ?points to check the possible values).
Also, you should scale the pt.cex values in order to make the bullets not too big for the legend, and use pt.bg to set the background color of the bullets, e.g.

# scaled between 1 and 2
scaled <- 1 + ((2-1) * (size_vec - min(size_vec) ) / (  max(size_vec) - min(size_vec) ) )
legend('topleft',legend=levels(sizeCut),pt.cex=scaled,col='black',pch=21, pt.bg='orange')

EDIT :

Unfortunately, calculating the right sizes of the bullets is not easy; a possible workaround is plotting white bullets then manually add the vertices to the legend in the same way they are plotted inside the plot.igraph function :

# N.B. /200 is taken from plot.igraph source code
a <- legend('topleft',legend=levels(sizeCut),pt.cex=size_vec/200,col='white',
            pch=21, pt.bg='white')
x <- (a$text$x + a$rect$left) / 2
y <- a$text$y
symbols(x,y,circles=size_vec/200,inches=FALSE,add=TRUE,bg='orange')

Disclaimer: this code heavily relies on the source code of plot.igraph function that might be changed in a future version of igraph. Probably you should search for another plot function for igraph which natively allows to add a legend.