Three ways of plotting a network graph of nodes with geographic coordinates on a map

networkmap.R

# Plot a network graph of nodes with geographic coordinates on a map.
#
# Author: Markus Konrad <markus.konrad@wzb.eu>
# May 2018
#
# This script shows three ways of plotting a network graph on a map.
# The following information should be visualized (with the respective
# aestethics added):
#
# * graph nodes with:
#   * position on map -> x,y position of the node
#   * node weight (degree of the node) -> node size
#   * node label -> also x,y position of the node
# * edges between nodes with:
#   * edge weight -> edge width
#   * edge category -> edge color

library(assertthat)
library(dplyr)
library(purrr)
library(igraph)
library(ggplot2)
library(ggraph)
library(ggmap)

# -------------------------------------- #
# Preparation: generate some random data #
# -------------------------------------- #

set.seed(123)

N_EDGES_PER_NODE_MIN <- 1
N_EDGES_PER_NODE_MAX <- 4
N_CATEGORIES <- 4

country_coords_txt <- "
 1     3.00000  28.00000       Algeria
 2    54.00000  24.00000           UAE
 3   139.75309  35.68536         Japan
 4    45.00000  25.00000 'Saudi Arabia'
 5    9.00000   34.00000       Tunisia
 6     5.75000  52.50000   Netherlands
 7   103.80000   1.36667     Singapore
 8   124.10000  -8.36667         Korea
 9    -2.69531  54.75844            UK
10    34.91155  39.05901        Turkey
11  -113.64258  60.10867        Canada
12    77.00000  20.00000         India
13    25.00000  46.00000       Romania
14   135.00000 -25.00000     Australia
15    10.00000  62.00000        Norway"

# nodes come from the above table and contain geo-coordinates for some
# randomly picked countries
nodes <- read.delim(text = country_coords_txt, header = FALSE,
                    quote = "'", sep = "",
                    col.names = c('id', 'lon', 'lat', 'name'))

# edges: create random connections between countries (nodes)
edges <- map_dfr(nodes$id, function(id) {
  n <- floor(runif(1, N_EDGES_PER_NODE_MIN, N_EDGES_PER_NODE_MAX+1))
  to <- sample(1:max(nodes$id), n, replace = FALSE)
  to <- to[to != id]
  categories <- sample(1:N_CATEGORIES, length(to), replace = TRUE)
  weights <- runif(length(to))
  data_frame(from = id, to = to, weight = weights, category = categories)
})

edges <- edges %>% mutate(category = as.factor(category))

# create the igraph graph object

g <- graph_from_data_frame(edges, directed = F, vertices = nodes)

# --------------------------------------------------------------------- #
# Common data structures and ggplot objects for all the following plots #
# --------------------------------------------------------------------- #

# create a data frame for plotting the edges
# join with nodes to get start and end positions for each
# edge (x, y and xend, yend)

edges_for_plot <- edges %>%
  inner_join(nodes %>% select(id, lon, lat), by = c('from' = 'id')) %>%
  rename(x = lon, y = lat) %>%
  inner_join(nodes %>% select(id, lon, lat), by = c('to' = 'id')) %>%
  rename(xend = lon, yend = lat)

assert_that(nrow(edges_for_plot) == nrow(edges))

# use the node degree for scaling the node sizes
nodes$weight = degree(g)

# common plot theme
maptheme <- theme(panel.grid = element_blank()) +
  theme(axis.text = element_blank()) +
  theme(axis.ticks = element_blank()) +
  theme(axis.title = element_blank()) +
  theme(legend.position = "bottom") +
  theme(panel.grid = element_blank()) +
  theme(panel.background = element_rect(fill = "#596673")) +
  theme(plot.margin = unit(c(0, 0, 0.5, 0), 'cm'))

# common polygon geom for plotting the country shapes
country_shapes <- geom_polygon(data = map_data('world'), aes(x = long, y = lat, group = group),
                               fill = "#CECECE", color = "#515151", size = 0.15)
# common coordinate system for all the following plots
mapcoords <- coord_fixed(xlim = c(-150, 180), ylim = c(-55, 80))

# ------------------------------- #
# Solution 1: ggplot + ggmap only #
# ------------------------------- #

# try to plot with scaled edge widths and node sizes
# this will fail because we can only use the "size" aesthetic twice

ggplot(nodes) + country_shapes +
  geom_curve(aes(x = x, y = y, xend = xend, yend = yend,     # draw edges as arcs
                 color = category, size = weight),
             data = edges_for_plot, curvature = 0.33, alpha = 0.5) +
  scale_size_continuous(guide = FALSE, range = c(0.25, 2)) + # scale for edge widths
  geom_point(aes(x = lon, y = lat, size = weight),           # draw nodes
             shape = 21,   
             fill = 'white', color = 'black', stroke = 0.5) +
  scale_size_continuous(guide = FALSE, range = c(1, 6)) +    # scale for node size
  geom_text(aes(x = lon, y = lat, label = name),             # draw text labels
            hjust = 0, nudge_x = 1, nudge_y = 4,
            size = 3, color = "white", fontface = "bold") +
  mapcoords + maptheme

# Results in warning: "Scale for 'size' is already present. Adding another scale for
# 'size', which will replace the existing scale."

# now a plot with static node size:

ggplot(nodes) + country_shapes +
  geom_curve(aes(x = x, y = y, xend = xend, yend = yend,     # draw edges as arcs
                 color = category, size = weight),
             data = edges_for_plot, curvature = 0.33, alpha = 0.5) +
  scale_size_continuous(guide = FALSE, range = c(0.25, 2)) + # scale for edge widths
  geom_point(aes(x = lon, y = lat),                          # draw nodes
             shape = 21, size = 3,
             fill = 'white', color = 'black', stroke = 0.5) +
  geom_text(aes(x = lon, y = lat, label = name),             # draw text labels
            hjust = 0, nudge_x = 1, nudge_y = 4,
            size = 3, color = "white", fontface = "bold") +
  mapcoords + maptheme


# ------------------------------------ #
# Solution 2: ggplot2 + ggmap + ggraph #
# ------------------------------------ #

# prepare layout: use "manual" layout with geo-coordinates
node_pos <- nodes %>% select(lon, lat) %>% rename(x = lon, y = lat)
lay <- create_layout(g, 'manual', node.positions = node_pos)
assert_that(nrow(lay) == nrow(nodes))

# use the node degree for scaling the node sizes
lay$weight <- degree(g)

ggraph(lay) + country_shapes +
  geom_edge_arc(aes(color = category, edge_width = weight,   # draw edges as arcs
                    circular = FALSE),
                data = edges_for_plot, curvature = 0.33, alpha = 0.5) +
  scale_edge_width_continuous(range = c(0.5, 2),             # scale for edge widths
                              guide = FALSE) +
  geom_node_point(aes(size = weight), shape = 21,            # draw nodes
                  fill = "white", color = "black",
                  stroke = 0.5) +
  scale_size_continuous(range = c(1, 6), guide = FALSE) +    # scale for node widths
  geom_node_text(aes(label = name), repel = TRUE, size = 3,
                 color = "white", fontface = "bold") +
  mapcoords + maptheme


# --------------------------------------------------------------- #
# Solution 3: the hacky way (overlay several ggplot "plot grobs")
# --------------------------------------------------------------- #

theme_transp_overlay <- theme(
  panel.background = element_rect(fill = "transparent", color = NA),
  plot.background = element_rect(fill = "transparent", color = NA)
)

# the base plot showing only the world map
p_base <- ggplot() + country_shapes + mapcoords + maptheme

# first overlay: edges as arcs
p_edges <- ggplot(edges_for_plot) +
  geom_curve(aes(x = x, y = y, xend = xend, yend = yend,     # draw edges as arcs
                 color = category, size = weight),
             curvature = 0.33, alpha = 0.5) +
  scale_size_continuous(guide = FALSE, range = c(0.5, 2)) +  # scale for edge widths
  mapcoords + maptheme + theme_transp_overlay +
  theme(legend.position = c(0.5, -0.1), legend.direction = "horizontal")

# second overlay: nodes as points
p_nodes <- ggplot(nodes) +
  geom_point(aes(x = lon, y = lat, size = weight),
             shape = 21, fill = "white", color = "black",    # draw nodes
             stroke = 0.5) +
  scale_size_continuous(guide = FALSE, range = c(1, 6)) +    # scale for node size
  geom_text(aes(x = lon, y = lat, label = name),             # draw text labels
            hjust = 0, nudge_x = 1, nudge_y = 4,
            size = 3, color = "white", fontface = "bold") +
  mapcoords + maptheme + theme_transp_overlay

# combine the overlays to a full plot

# proper positioning of the grobs can be tedious... I found that
# using `ymin` works quite well but manual tweeking of the
# parameter seems necessary

p <- p_base +
  annotation_custom(ggplotGrob(p_edges), ymin = -74) +
  annotation_custom(ggplotGrob(p_nodes), ymin = -74)

print(p)

2022 here: For anyone else getting an error on node.positions, it seems to be a breaking change since this version (from 2018). I changed "node.positions" to "layout" in the code which helped.

Another error for the ggraph (method number 2 here) also gave an error that it could not find the object "edge.id". I just added that column manually to the edges_for_plot dataframe before running the geom_edge_arc like this:

edges_for_plot$edge.id <- c(1:38) #so now it has a column edge.id

Then ran the final ggraph plot:
ggraph(lay) + country_shapes +
geom_edge_arc(aes(color = category, edge_width = weight, # draw edges as arcs
circular = FALSE),
data = edges_for_plot, strength = 0.33, ###curvature is apparently changed to strength in the new ggraph version
alpha = 0.5) # and you can add the rest of the options in the original code here

Very helpful! Is there a way to control the position of arrows, say put it at the middle of a line? This can avoid too many arrows overlaying with each other when they point to the same location. Thanks!