A cartogram is a map in which the geometry of regions is distorted in order to convey the information of an alternate variable. In this post, we are going to draw a map of Africa where the size of each country is distorted proportionally to its population.
First of all, you need to understand what a geospatial object is, and how to plot it with
R. See the background map section of the gallery!
Let’s use the
maptools library which provides a geospatial object with the Africa’s boundaries. Note that you can get a similar object from a shapefile, or from a geojson file!
We can plot the boundaries using the
The geospatial object has a
data slot: an attached data frame that provides several information for each region. It notably gives the population of each country in 2005.
We can thus use the
cartogram library to distort the size of each country, proportionally to this column. The new geospatial object we get can be draw with the same
See how Nigeria is now bigger?
Cartogram are very often used in combination with choropleth map. Since we have a spatial object and a numeric value associated with each region, it is possible to color each region according to its value.
Let’s use ggplot2 to add some color, title, legend, background and so on. We now have a nice cartogram chloropleth map of Africa!
# It is a new geospatial object: we can use all the usual techniques on it! Let's start with a basic ggplot2 chloropleth map: library(tidyverse) library(broom) spdf_fortified <- tidy(afr_cartogram) spdf_fortified = spdf_fortified %>% left_join(. , afr_cartogram@data, by=c("id"="ISO3")) ggplot() + geom_polygon(data = spdf_fortified, aes(fill = POP2005, x = long, y = lat, group = group) , size=0, alpha=0.9) + coord_map() + theme_void()
# As seen before, we can do better with a bit of customization library(viridis) ggplot() + geom_polygon(data = spdf_fortified, aes(fill = POP2005/1000000, x = long, y = lat, group = group) , size=0, alpha=0.9) + theme_void() + scale_fill_viridis(name="Population (M)", breaks=c(1,50,100, 140), guide = guide_legend( keyheight = unit(3, units = "mm"), keywidth=unit(12, units = "mm"), label.position = "bottom", title.position = 'top', nrow=1)) + labs( title = "Africa 2005 Population" ) + ylim(-35,35) + theme( text = element_text(color = "#22211d"), plot.background = element_rect(fill = "#f5f5f4", color = NA), panel.background = element_rect(fill = "#f5f5f4", color = NA), legend.background = element_rect(fill = "#f5f5f4", color = NA), plot.title = element_text(size= 22, hjust=0.5, color = "#4e4d47", margin = margin(b = -0.1, t = 0.4, l = 2, unit = "cm")), legend.position = c(0.2, 0.26) ) + coord_map()