ggplot and maps: how to bold borders of the regions

Question

I'm trying to draw map using a shapefile. The shapefile (prov2022) is this

https://easyupload.io/j8bdaf

This is a screenshot of it

COD_REG = is the code of the regions.

COD_PROV = is the code of the provices.

Here an example of the dataset instead

COD_PROV     Province               Real wage 
1            Torino                 1962,18
6            Alessandria            1742,85
5            Asti                   1541,81
96           Biella                 1612,2
4            Cuneo                  1574
3            Novara                 1823,53
103          Verbano -Cusio-Ossola  1584,49
2            Vercelli               1666,21
7            Aosta                  1747,81
10           Genova                 2066,42
8            Imperia                1498,01
11           La Spezia              1871,34
9            Savona                 1770,41
15           Milano                 2240,03
16           Bergamo                1729,17
17           Brescia                1773,38
13           Como                   1832,57

I'm drawing the map using this script ( which works perfectly)

right_join(prov2022, dataset, by = "COD_PROV") %>% 
  ggplot(aes(fill = `Real wage`)) +
  geom_sf() +
  theme_void() +
  theme(legend.title=element_blank())+
  scale_fill_gradientn(colors = c( "#FFFFFF","#FFFF00", "#FF0000", "#000000"))

The map I get is divided inside according with the borders of each province. Is there a way to draw in bold also the geographical boundaries of the regions?? ( each region has inside 6/7 provinces)

Answer 1

If you don't have the shapefile info on each region (i.e. one big geometry that includes all of its provinces), then it's best to create those for each region first in a separate dataframe. Then you can easily plot them as another layer (with a thicker 'linewidth' argument) to get your desired effect. e.g.

regions <- prov2022 %>%
  group_by(COD_REG) %>% 
  summarize(geometry = st_union(geometry))

right_join(prov2022, dataset, by = "COD_PROV") %>% 
  ggplot() +
  geom_sf(aes(fill = `Real wage`) +
  theme_void() +
  theme(legend.title=element_blank())+
  scale_fill_gradientn(colors = c( "#FFFFFF","#FFFF00", "#FF0000", "#000000")) +
  geom_sf(data = regions, aes(geometry = geometry), linewidth = 2, fill = NA)