Setting subgraph/cluster attributes in Rgraphviz - r

Question

I want to plot a graph via Rgraphviz but I can't handle the design attributes of the clusters that I set.
There are similar questions already on SO and elsewhere but none has a real minimal working example and none of them is answered. So I want to try to ask a complete question to receive a complete answer. As an introduction to the package, I read the paper "How To Plot A Graph Using Rgraphviz" by Gentry, Gentleman, and Huber.
My example network:
library(Rgraphviz)
set.seed(123)
V <- letters[1:6]
M <- 1:4
g1 <- randomGraph(V, M, 0.2)
If I want to plot it, I can easily give it some attributes via a list:
attributes <- list(node = list(shape = "rectangle", fixedsize = FALSE),
graph = list(layout = "dot", bgcolor = "transparent"))
plot(g1, attrs = attributes )
Plotting it via plot(g1) gives the following result:
Now I want to define two clusters/subgraphs. This can be done this way:
sg1= subGraph(c("a", "e", "f"), g1)
sg2= subGraph(c("b", "c", "d"), g1)
subGList <- vector(mode = "list", length = 2)
subGList[[1]] <- list(graph = sg1, cluster = TRUE)
subGList[[2]] <- list(graph = sg2, cluster = TRUE)
Plotting the graph again now including a subGlist argument:
plot(g1, attrs = attributes , subGList = subGList)
So, obviously, there has been a change in the setting and even though it would be convenient having the clusters a little bit more separated, the result is ok.
Now if I want to define cluster-specific styles or try to have them framed, I start having problems. According to page 4 of the mentioned introductory paper one can simply add an element called attrs to the sublists of subGlist.
To my understanding, it should work this way:
subGList[[1]] <- list(graph = sg1,
cluster = TRUE,
attrs = c(fontcolor = "red"))
plot(g1, attrs = attrs, subGList = subGList)
Unfortunately, it doesn't. As mentioned, I would like to frame my clusters (similar to this SO post) but as I can't even handle the fontcolors of the clusters, I think I make a somehow more fundamental mistake.
My complete code:
library(Rgraphviz)
set.seed(123)
V <- letters[1:6]
M <- 1:4
g1 <- randomGraph(V, M, 0.2)
attributes <- list(node = list(shape = "rectangle", fixedsize = FALSE),
graph = list(layout = "dot", bgcolor = "transparent"))
#plot(g1, attrs = attributes )
sg1= subGraph(c("a", "e", "f"), g1)
sg2= subGraph(c("b", "c", "d"), g1)
subGList <- vector(mode = "list", length = 2)
subGList[[1]] <- list(graph = sg1, cluster = TRUE)
subGList[[2]] <- list(graph = sg2, cluster = TRUE)
#plot(g1, attrs = attributes , subGList = subGList)
subGList[[1]] <- list(graph = sg1,
cluster = TRUE,
attrs = c(fontcolor = "red"))
plot(g1, attrs = attrs, subGList = subGList)
I hope someone can help! Thank you