[igraph] subgraph and plot

[simone gabbriellini]

Hi List,

I am trying to plot a graph with grey vertices and then to color some  
of them with different colors, due to core/periphery structure I found  
with Ucinet... (btw, how could it be wonderful to have a core/ 
periphery function in igraph... :)

this is the code I use:

g <- read.graph(nomefile, format="graphml")
g <- delete.vertices(g, which(degree(g)==0)-1)
g<-simplify(g)
#
ll<-layout.fruchterman.reingold(g, weights=E(g)$weigths)
ll<-layout.norm(ll,-1,1,-1,1)
#
plot(g, layout=ll, vertex.size=5, vertex.label=V(g)$id,  
vertex.label.dist=0.4,
        vertex.label.cex=0.4, vertex.color="grey", vertex.label.color="black",
	vertex.frame.color="black", edge.color="lightgrey",  
edge.arrow.size=0.3, rescale=FALSE, main="March 2004")
#
#SWG
core<-c("Brenn Tantor", "Kveldersen")
corenumber<-array(length(core))
for (i in 1:length(core)){
        corenumber[i]<-V(g)[label==core[i]]
        }
g4 <- subgraph(g, corenumber)
g4 <- delete.edges(g4, E(g4))        
l4 <- ll[ corenumber+1, ]
plot(g4, layout=l4, vertex.size=5, vertex.label=NA,
        vertex.color=NA, vertex.frame.color="blue", add=TRUE, rescale=FALSE)

and it works perfect untill the "core" variable contains at least two  
names... if it happens to be only one name, then I am able to  
subgraph, to find the layout coordinates of that single node, but when  
I try to plot I get this error from layout:

Error in layout[, 1] : wrong number of dimensions

can someone help me?
thanks,
simone