Changing node/vertice opacity in iGraph in R

Question

I have a network that, when I plot it, has a number of overlapping nodes. I want to change the opacity of the colors so that you can see nodes underneath others when they overlap. As an example, see this video: https://vimeo.com/52390053

I'm using iGraph for my plots. Here's a simplified blurb of code:

net1 <- graph.data.frame(myedgelist, vertices=nodeslist, directed = TRUE)

g <- graph.adjacency(get.adjacency(net1))

V(g)$color <- nodeslist$colors  #This is a set of specific colors corresponding to each node. They are in the format "skyblue3". (These plot correctly for me). 

E(g)$color <-"gray" 

plot.igraph(g)

I can't, however, find an option in iGraph to change the opacity of the node colors.

Any idea how I might do this? I thought maybe something like V(g)$alpha <- 0.8, but this doesn't do anything.

Answer 1

You might wanna try e.g. this:

library(igraph)
set.seed(1)
g <- barabasi.game(200)
plot(g, 
     vertex.color = adjustcolor("SkyBlue2", alpha.f = .5), 
     vertex.label.color = adjustcolor("black", .5))

Answer 2

A way I find easier to control than the method provided by lukeA is to use rgb(). You can specify color (of a node, node frame, edge, etc.) in terms of its four channels: R, G, B, and A (alpha):

library(igraph)
set.seed(1)
g <- barabasi.game(200)
plot(g, 
     vertex.color = rgb(0,0,1,.5), 
     vertex.label.color = rgb(0,0,0,.5))

Another advantage is you can easily vary alpha (or color) according to a vector. The example below isn't exactly practical, but you get the idea how this could be used:

library(igraph)
set.seed(1)
g <- barabasi.game(200)

col.vec <- runif(200,0,1)
alpha.vec <- runif(200,0,1)

plot(g, 
     vertex.color = rgb(0,0,col.vec,alpha.vec), 
     vertex.label.color = rgb(0,0,0,.5))