chord diagram

R code of the ggplot2 example

library(ggplot2)
library(dplyr)
library(ggforce)
library(migest)

## Migrants between the five Italian macro-regions in 2000, all ages.
flows <- italy_area |>
  filter(year == 2000, orig != dest) |>
  group_by(orig, dest) |>
  summarise(flow = sum(flow) / 1000, .groups = "drop")
regions <- c("North-West", "North-East", "Center", "South", "Islands")

## Each region gets an arc as long as its departures plus its arrivals,
## departures first; angles in radians, clockwise from the top (ggforce).
gap <- 0.06
scale <- (2 * pi - gap * length(regions)) / (2 * sum(flows$flow))
sectors <- tibble(region = regions) |>
  mutate(out = sapply(region, \(r) sum(flows$flow[flows$orig == r])),
         inn = sapply(region, \(r) sum(flows$flow[flows$dest == r])),
         end = cumsum((out + inn) * scale + gap) - gap, start = end - (out + inn) * scale)
flows <- flows |>
  mutate(orig = factor(orig, regions), dest = factor(dest, regions)) |>
  arrange(orig, dest) |>
  group_by(orig) |>
  mutate(a2 = sectors$start[as.integer(orig)] + cumsum(flow) * scale, a1 = a2 - flow * scale) |>
  arrange(dest, orig) |>
  group_by(dest) |>
  mutate(b2 = sectors$start[as.integer(dest)] + (sectors$out[as.integer(dest)] + cumsum(flow)) * scale,
         b1 = b2 - flow * scale) |>
  ungroup()

## A ribbon: the arc it leaves, a curve through the centre, the arc it
## reaches, a curve back.
arc <- function(from, to, r = 1) {
  a <- seq(from, to, length.out = 20)
  cbind(r * sin(a), r * cos(a))
}
bend <- function(p, q) {
  t <- seq(0, 1, length.out = 30)
  (1 - t)^2 %o% p + (t^2) %o% q
}
ribbons <- bind_rows(lapply(seq_len(nrow(flows)), function(i) {
  f <- flows[i, ]
  A <- arc(f$a1, f$a2)
  B <- arc(f$b1, f$b2)
  xy <- rbind(A, bend(A[20, ], B[1, ]), B, bend(B[20, ], A[1, ]))
  tibble(id = i, orig = f$orig, x = xy[, 1], y = xy[, 2])
}))

p <- ggplot() +
  geom_polygon(data = ribbons, aes(x = x, y = y, group = id, fill = orig), alpha = 0.65) +
  geom_arc_bar(data = sectors, aes(x0 = 0, y0 = 0, r0 = 1.02, r = 1.1, start = start, end = end,
                                   fill = factor(region, regions)), colour = NA) +
  geom_text(data = sectors, aes(x = 1.22 * sin((start + end) / 2), y = 1.22 * cos((start + end) / 2),
                                label = region, hjust = ifelse(sin((start + end) / 2) > 0, 0, 1)),
            size = 2.8, colour = "#262626") +
  scale_fill_manual(values = setNames(lex_pal_discrete(5), regions)) +
  coord_equal(xlim = c(-2.1, 2.1), ylim = c(-1.25, 1.25), clip = "off") +
  labs(title = "The South sent the most migrants to other regions",
       subtitle = "Moves between Italian regions in 2000, colored by origin",
       x = NULL, y = NULL,
       caption = "Data: ISTAT, via the migest R package") +
  theme_lex_viz(grid = "", arrows = "") +
  theme(legend.position = "none", axis.text = element_blank())