Remove border lines in ggplot map/choropleth

Question

I would like to remove the lines between regions of a choropleth generated in ggplot. My question is motivated by a very large map with very very small regions

Answer 1

Another option is to set both fill and color equal to group, which worked on the macOS I tried it on:

library("ggplot2")
library("maps")
tn = map_data("county", region = "tennessee")
ggplot(tn, aes(x = long, y = lat, group = group)) + 
  geom_polygon(aes(fill = group, color = group))

Output: