Using cartograms with parlitools

Evan Odell

2022-01-31

Scaling with cartogram

You can use the cartogram package with parlitools to produce maps with scaled areas, as seen in Xinye Li’s post on Brexit votes.

As the cartogram package requires SpatialPolygonsDataFrame objects, instead of simple features objects, the west_hex_map object is converted first to a Spatial object, and then to a SpatialPolygonsDataFrame object, as there is no function to convert directly from sf to SpatialPolygonsDataFrame.

Scaling by marginality

This map scales constituencies based on how marginal they are, by subtracting the majority (the difference in votes between the winning and second place candidate) from 100 and cubing that value. Larger constituencies are those deemed to be more marginal.

It uses datasets included in this package, namely the British Election Study data of the 2017 General Election supplemented with the Northern Ireland results (bes_2017), party_colours and west_hex_map.

library(leaflet)
library(sf)
library(htmlwidgets)
library(dplyr)
library(parlitools)
library(cartogram)

west_hex_map <- parlitools::west_hex_map

party_colour <- parlitools::party_colour

elect2017 <- parlitools::bes_2017

#Join to current MP data
elect2017_win_colours <- left_join(elect2017, party_colour, by = c("winner_17" ="party_name")) 

#Join colours to hexagon map
gb_hex_map <- right_join(west_hex_map, elect2017_win_colours,
                         by = c("gss_code"="ons_const_id"))

# gb_hex_map <- as(gb_hex_map, "Spatial")
# 
# gb_hex_map <- as(gb_hex_map, "SpatialPolygonsDataFrame")

gb_hex_map$majority_17 <- round(gb_hex_map$majority_17, 2)

gb_hex_map$turnout_17 <- round(gb_hex_map$turnout_17, 2)

gb_hex_map$marginality <- (100-gb_hex_map$majority_17)^3

gb_hex_map <- st_transform(gb_hex_map, "+init=epsg:3395")

gp_hex_scaled <- cartogram_cont(gb_hex_map, 'marginality', itermax = 5)

gp_hex_scaled <- st_transform(gp_hex_scaled, "+init=epsg:4326")

# Creating map labels
labels <- paste0(
  "Constituency: ", gp_hex_scaled$constituency_name.y, "</br>",
  "Most Recent Winner: ", gp_hex_scaled$winner_17, "</br>",
  "Most Recent Majority: ", gp_hex_scaled$majority_17, "%","</br>",
  "Turnout: ", gp_hex_scaled$turnout_17, "%"
) %>% lapply(htmltools::HTML)

# Creating the map itself
leaflet(options=leafletOptions(
  dragging = FALSE, zoomControl = FALSE, tap = FALSE,
  minZoom = 6, maxZoom = 6, maxBounds = list(list(2.5,-7.75),list(58.25,50.0)),
  attributionControl = FALSE),
  gp_hex_scaled) %>%
  addPolygons(
    color = "grey",
    weight=0.75,
    opacity = 0.5,
    fillOpacity = 1,
    fillColor = ~party_colour,
    label=labels) %>% 
  htmlwidgets::onRender(
    "function(x, y) {
        var myMap = this;
        myMap._container.style['background'] = '#fff';
    }")%>% 
  mapOptions(zoomToLimits = "first")