Plotting neighborhoods network to a ggplot maps

Question

How to insert spatial network plot to a ggplot

library(rgeos) 
library(rgdal)
library(dplyr)
require(maps)
require(viridis)
library(ggplot2)
library(spdep)


some.eu.countries <- c(
  "Portugal", "Spain", "France", "Switzerland", "Germany",
  "Austria", "Belgium", "UK", "Netherlands",
  "Denmark", "Poland", "Italy", 
  "Croatia", "Slovenia", "Hungary", "Slovakia",
  "Czech republic"
)
# Retrievethe map data

some.eu.maps <- map_data("world", region = some.eu.countries)

# Compute the centroid as the mean longitude and lattitude
region.lab.data <- some.eu.maps %>%
  group_by(region) %>%
  summarise(long = mean(long), lat = mean(lat))

cns <- knearneigh(cbind(region.lab.data$long, region.lab.data$lat), k=3, longlat=T) 
scnsn <- knn2nb(cns, row.names = NULL, sym = T) 
cns
scnsn
cS <- nb2listw(scnsn) 
cS
summary(cS)

# Plotting neighbours network
plot(cS, cbind(region.lab.data$long, region.lab.data$lat))


# Now plotting countries
ggplot(some.eu.maps, aes(x = long, y = lat)) +
  geom_polygon(aes( group = group, fill = region), colour = "black")+
  geom_point(aes(region.lab.data$long, region.lab.data$lat), data = region.lab.data, size = 6)

I would like to insert #Plotting neighbours network to a # Now plotting countries in another words I would like to have a ggplot of countries and also see network of neighbours in it.

Answer 1

I looked under plot.nb, which is the plot method of your nb2listw output, and i think it basically makes a segment for all connections between your input.

# Now plotting countries
g = ggplot(some.eu.maps, aes(x = long, y = lat)) +
  geom_polygon(aes( group = group, fill = region), colour = "black")+
  geom_point(aes(region.lab.data$long, region.lab.data$lat), data = region.lab.data, size = 6)

# take out the connections from your nb object
# and assign them the lat and long in a dataframe
n = length(attributes(cS$neighbours)$region.id)
DA = data.frame(
from = rep(1:n,sapply(cS$neighbours,length)),
to = unlist(cS$neighbours),
weight = unlist(cS$weights)
)
DA = cbind(DA,region.lab.data[DA$from,2:3],region.lab.data[DA$to,2:3])
colnames(DA)[4:7] = c("long","lat","long_to","lat_to")
#plot it using geom_segment
g + geom_segment(data=DA,aes(xend=long_to,yend=lat_to),size=0.3,alpha=0.5)

This is what I got (caveat, I don't know what to do with the weights):