There are a few similar posts (like this one and this too), but my question might be more basic for the igraph package functionality.

This is the minimal example that generates a simple graph:

library(MASS)
library(pcalg)
library(igraph)

# Some variance-covariance
Corrs <- matrix(c(1.0,0.6,0.7,0.5,0.6,1.0,0.5,0.6,0.7,0.5,1.0,0.7,0.5,0.6,0.7,1.0), 4, 4)
SDs <- c(1.0,0.5,2.0,1.0)
Covs <- SDs %*% t(SDs) * Corrs

dat = as.data.frame(mvrnorm(100, mu=c(0,0,0,0), Sigma=Covs))

n = nrow(dat)
V = colnames(dat) # node names
pc1 = pc(suffStat=list(C=cor(dat), n=n),
    indepTest=gaussCItest, # indep.test: partial correlations
    alpha=0.05, labels=V, u2pd='retry')

gr = graph_from_graphnel(pc1@graph)

plot.igraph(gr, layout=layout_in_circle,
    vertex.label=V, vertex.shape='circle', vertex.size=30,
    vertex.label.cex=0.55, vertex.label.color='black',
    edge.arrow.size=0.6)

If I try to add an edge (for example, V2 —> V3) it ends with only that edge while others, “original” edges are gone:

gr = graph_from_graphnel(pc1@graph) + edge(2, 3, color='blue')

plot.igraph(gr, layout=layout_in_circle,
    vertex.label=V, vertex.shape='circle', vertex.size=30,
    vertex.label.cex=0.55, vertex.label.color='black',
    edge.arrow.size=0.6)

I also tried with the add_edges() function and got the same result.

So, how can I add an edge to the existing graph? And how can I mark that particular edge with different color (which I managed) and lty?