In r-tmap/tmap: Thematic Maps

knitr::opts_chunk$set(
  collapse = TRUE,
  fig.width = 8,
  comment = "#>"
)
hook_output <- knitr::knit_hooks$get("output")
knitr::knit_hooks$set(output = function(x, options) {
   lines <- options$output.lines
   if (is.null(lines)) {
     return(hook_output(x, options))  # pass to default hook
   }
   x <- unlist(strsplit(x, "\n"))
   more <- "..."
   if (length(lines)==1) {        # first n lines
     if (length(x) > lines) {
       # truncate the output, but add ....
       x <- c(head(x, lines), more)
     }
   } else {
     x <- c(more, x[lines], more)
   }
   # paste these lines together
   x <- paste(c(x, ""), collapse = "\n")
   hook_output(x, options)
 })

library(tmap)
library(tmap.cartogram)
tmap_options(scale = 0.75)

Cartograms

In cartograms, areal regions are distorted such that the obtained area sizes are proportional to a quantitative variable. There are three types: contiguous, non-contiguous and dorling cartogram.

The extension package tmap.cartogram provides new layer functions for tmap to create various types of cartograms.

It is strongly recommended to use an equal area crs (see instructions).

Contiguous cartogram

The shapes of the polygons are distorted, where the neighborhood relationships between them are preserved as much as possible.

Africa = World[World$continent == "Africa", ]

tm_shape(Africa, crs = "+proj=robin") +
    tm_cartogram(size = "pop_est", 
                 fill = "life_exp",
                 fill.scale = tm_scale_intervals(values = "-cols4all.bu_br_div"),
                 fill.legend = tm_legend("Age"),
                 options = opt_tm_cartogram(itermax = 15)) +
tm_title("Life Expectancy")

Non-contiguous cartograms

Here, the polygons are only resized while they keep their shape.

tm_shape(Africa, crs = "+proj=robin") +
    tm_cartogram_ncont(size = "pop_est", 
                       fill = "inequality",
                       fill.scale = tm_scale_continuous(values = "cols4all.pu_gn_div", values.range = c(0, 0.5)),
                       fill.legend = tm_legend(""),
                       options = opt_tm_cartogram_ncont()) +
    tm_text("name", options = opt_tm_text(point.label = TRUE)) +
tm_title("Income inequality (Gini coefficient)")

Dorling cartogram

Polygons are replaced by non-overlapping bubbles.

tm_shape(World, crs = "+proj=robin") +
    tm_cartogram_dorling(size = "pop_est", 
                         fill = "press",
                         fill.scale = tm_scale_continuous(values = "cols4all.pu_gn_div", midpoint = 50),
                         fill.legend = tm_legend("", height = 30)) +
tm_title("World Press Freedom Index")