adjacency matrix

R code of the ggplot2 example

library(ggplot2)
library(dplyr)
library(igraph)

## Ties between the 34 members of a karate club: one row and one column per
## member, a square wherever two members met outside the club (each tie is
## drawn twice, the matrix is symmetric). Members are ordered by the faction
## they joined when the club split, which does not come from the ties.
data("karate", package = "igraphdata")
club <- upgrade_graph(karate)
members <- tibble(id = seq_len(vcount(club)),
                  faction = ifelse(V(club)$Faction == 1, "Mr Hi's faction", "John A's faction"),
                  degree = degree(club)) |>
  arrange(faction != "Mr Hi's faction", desc(degree)) |>
  mutate(position = row_number())
ends <- as_edgelist(club, names = FALSE)
ties <- tibble(a = c(ends[, 1], ends[, 2]), b = c(ends[, 2], ends[, 1])) |>
  mutate(x = members$position[match(a, members$id)], y = members$position[match(b, members$id)],
         fa = members$faction[match(a, members$id)], fb = members$faction[match(b, members$id)],
         tie = ifelse(fa == fb, fa, "Between factions"))
blocks <- members |>
  group_by(faction) |>
  summarise(from = min(position) - 0.5, to = max(position) + 0.5)

p <- ggplot(ties, aes(x = x, y = y)) +
  geom_rect(data = blocks, aes(xmin = from, xmax = to, ymin = from, ymax = to, fill = faction),
            alpha = 0.12, inherit.aes = FALSE) +
  geom_tile(aes(fill = tie), width = 0.85, height = 0.85) +
  annotate("text", x = -0.5, y = (blocks$from + blocks$to) / 2, label = sub(" faction", "\nfaction", blocks$faction),
           colour = unname(lex_colors[c("blue", "plum")]), hjust = 1, size = 3, lineheight = 0.9) +
  scale_fill_manual(values = c(`Mr Hi's faction` = lex_colors[["plum"]],
                               `John A's faction` = lex_colors[["blue"]],
                               `Between factions` = "#a6a6a6")) +
  scale_y_reverse() +
  coord_fixed(clip = "off") +
  labs(title = "68 of the 78 ties join members of the same faction",
       subtitle = "Karate club, 1970–1972: members ordered by the faction they joined when the club split; gray: ties between factions",
       caption = "Data: Zachary 1977, via the igraphdata R package") +
  theme_lex_viz(grid = "", arrows = "") +
  theme(legend.position = "none", axis.text = element_blank(), axis.title = element_blank(),
        panel.border = element_rect(colour = "#d6d6d6", fill = NA, linewidth = 0.3))