Plot map with values for countries as color in R?

Question

I have following simple example data which I want to plot on a map with gradient color corresponding to value of the given country.

ddf = read.table(text="
country value
USA 10
UK 30
Sweden 50
Japan 70
China 90
Germany 100
France 80
Italy 60
Nepal 40
Nigeria 20
", header=T)

On google search, I found several sites. However, I am looking for code which is small and clear, and should preferably be fast (I found ggplot methods to be relativley slow). The resolution of world map need not be high.

I tried following code:

library(maptools)
data(wrld_simpl)
plot(wrld_simpl)

Specific nations can be colored as given on : Using [R] maps package - colouring in specific nations on a world map Using the command:

plot(wrld_simpl, col = c(gray(.80), "red")[grepl("^U", wrld_simpl@data$NAME) + 1])

But how can I get map with above data in a gradient of colors. Thanks for your help.

Answer 1

I thought the other answers were slightly complicated, perhaps because this was asked/answered a relatively long time ago? Here's a simple way using ggplot2. I don't know what the benchmark is with respect to whether or not this is "fast".

library(ggplot2)
library(dplyr)

ddf = read.table(text="
country value
USA 10
UK 30
Sweden 50
Japan 70
China 90
Germany 100
France 80
Italy 60
Nepal 40
Nigeria 20
", header=T)

world <- map_data("world")

world %>%
  merge(ddf, by.x = "region", by.y = "country", all.x = T) %>%
  arrange(group, order) %>%
  ggplot(aes(x = long, y = lat, group = group, fill = value)) + geom_polygon()

This makes it easy to modify using all the common ggplot2 handles. For example, if we wanted to pretty this up quickly:

library(viridis)

world %>%
  merge(ddf, by.x = "region", by.y = "country", all.x = T) %>%
  arrange(group, order) %>%
  ggplot(aes(x = long, y = lat, group = group, fill = value)) +
  geom_polygon(color = "white", size = 0.2) +
  scale_fill_viridis("", na.value = "gray90") +
  theme_minimal() +
  theme(axis.text = element_blank(),
        axis.title = element_blank(),
        panel.grid = element_blank())

Answer 2

You could use rworldmap if you wanted less code and a coarser resolution map.

library(rworldmap)

#create a map-shaped window
mapDevice('x11')
#join to a coarse resolution map
spdf <- joinCountryData2Map(ddf, joinCode="NAME", nameJoinColumn="country")

mapCountryData(spdf, nameColumnToPlot="value", catMethod="fixedWidth")

Default categorisation, colours and legends can be altered, see this RJournal paper.

It would be faster with country codes rather than names.