12.3 Choropleth Maps

Sometimes maps use shading of polygons to display quantitative information about a geographic unit or qualitative information about what geographical units belong to different categories of a variable. We provide an example of this here.

• We are going to add a variable to our mapping data that we want to visualize
• We fill the plot using geom_polygon and can (optionally) indicate specific colors

usmap <- map_data("state")
usmap$nj <- ifelse(usmap$region == "new jersey", "Cannot turn left", 
                   "Can turn left")
usmap$nj <- as.factor(usmap$nj)

ggplot()+
  ## Note the fill= nj
  geom_polygon(data=usmap, aes(x=long, y=lat, group=group, fill=nj))+
  ## we can indicate colors for each category of the nj variable
  scale_fill_manual(values = c("gray", "red3"), name="Left Turns")+
  theme_void() +
  ggtitle("Geography of Left Turns")+
  coord_quickmap()