polygon - Keep customized colors with geom_polygon() - Geographic

r - color polygon boundaries by factor - Stack Overflow

Making Maps With R · Reproducible Research.

Remove lines from inside a polygon (shapefile) in r - Geographic Information Systems Stack Exchange

r - ggplot2 - how to fill nested polygons with colour? - Stack Overflow

Continuous colour gradient that applies to a single geom_polygon element with ggplot2 in R - Stack Overflow

Bubble map with ggplot2 – the R Graph Gallery

r - Trouble adding proportional circles on top of symbolized polygon in ggplot2 map - Geographic Information Systems Stack Exchange

r - plot small region of a large polygon map in ggplot2 - Stack Overflow

r - ggplot2 - how to fill nested polygons with colour? - Stack Overflow

Chapter 8 Creating Maps using ggplot2

Chapter 3 Data Visualization Exploring Data Science with R and the Tidyverse: A Concise Introduction

R/ggplot2 adding an additional line/polygon to a choropleth map - Geographic Information Systems Stack Exchange