How to change the border color of a polygon based on an attribute value in R tmap

Question

I am trying to create a map using R's tmap package that has a different color outline for each value of an attribute. When filling the polygons, for example, I can simply do tm_polygons(col = "variable") to get a map that fills the polygons based on the "variable" attribute. I wish to do a similar thing, but with just the border, not the fill. I'm not finding any good answers, so I was hoping to get some help here!

Here is an example that illustrates the idea:

library(tigris)
library(tmap)
library(tidyverse)

region_2 = states() %>% filter(REGION == 2)
states_of_interest = region_2 %>% filter(NAME %in% c("Ohio", "Missouri", "Wisconsin"))

tm_shape(region_2) + tm_polygons(col = "white") + tm_shape(states_of_interest) + .......

and I am not quite sure how to proceed

Basically, I want the borders of Ohio, Missouri, and Wisconsin to have a different color to distinguish them from one another, using the NAME attribute in states_of_interest. Any help would be greatly appreciated!

Answer 1

A potential workaround is to convert your POLYGONS to LINESTRING and proceed with tm_lines():

library(tigris)
#> To enable caching of data, set `options(tigris_use_cache = TRUE)`
#> in your R script or .Rprofile.
library(tmap)
library(tidyverse)

region_2 = states() %>% filter(REGION == 2)
#> Retrieving data for the year 2020
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states_of_interest = region_2 %>%
  filter(NAME %in% c("Ohio", "Missouri", "Wisconsin"))

states_lines <- states_of_interest %>% sf::st_cast("MULTILINESTRING")


tm_shape(region_2) + 
  tm_polygons(col = "white") +
  tm_shape(states_lines) +
  tm_lines(col="NAME", lwd=2)

^{Created on 2022-03-21 by the reprex package (v2.0.1)}