library(igraph)
g <- sample_islands(9, 20, 0.4, 9)
g <- simplify(g)
V(g)$grp <- as.character(rep(1:9, each = 20))
bb <- layout_as_backbone(g, keep = 0.4)
# add backbone links as edge attribute
E(g)$col <- FALSE
E(g)$col[bb$backbone] <- TRUE
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