polygon - Keep customized colors with geom_polygon() - Geographic

ggplot2 - producing heat map over Geo locations in R - Stack Overflow

polygon - Keep customized colors with geom_polygon() - Geographic Information Systems Stack Exchange

Chapter 14 Graphing Part II STA 444/5 - Introductory Data Science using R

Drawing beautiful maps programmatically with R, sf and ggplot2 — Part 2: Layers

Visualizing geo-spatial data with sf and plotly

raster - How to correctly plot SpatialPolygonDataFrames in ggplot and substract them in R - Geographic Information Systems Stack Exchange

Working with shapefiles, projections and world maps in ggplot

r - Colour the background of a ggplot polygon to match the lower value of the contour layer - Stack Overflow

Chapter 7 Map R Gallery Book

Making Maps with R - GeeksforGeeks

Custom color in a map(polygons) by ggplot in R and plot a legend - Stack Overflow

r - Why does geom_sf is not allowing fill with discrete column from a dataframe? - Stack Overflow

r - assign colors to geom_polygons where fill = discrete category - Stack Overflow