Following the reproducible example from HERE, what is the best way to add an edge (for example, V2 —> V3) to the `plot.igraph()`

, and that edge to be of different `color`

and `lty`

:

```
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 do this with the `edge()`

function, following some examples from the package itself, then only an added edge is presented. Thus, that edge is not **added** (I also tried with the `add_edges()`

function with the same result):

```
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)
```

Many thanks!