Building a heat world map

Question

I'm trying to build a world heat map and I'm stumped as to why it looked this:

My data is super simple:

Why is it not showing a normal map? Furthermore, the UK has the highest number, but it is grey?!

Please advise. Thanks

data <- data.frame(
  country = c("UK", "Ghana", "USA", "Thailand", "Columbia", "Brazil"),
  value = c(12, 2, 1, 2, 1, 1)
)

world <- map_data("world")
data <- merge(world, data, by.x = "region", by.y = "country")

ggplot(data, aes(x = long, y = lat, group = group)) +
  geom_polygon(aes(fill = value), color = "grey") +
  scale_fill_gradient(low = "blue", high = "red") +
  theme_void()

I've tried various different packages but haven't been able to solve the problem.

Answer 1

I get a plausible result with three changes:

1. replace merge with dplyr::left_join. I suspect the merge is not preserving the proper ordering of the polygon points, leading to crisscrossing between perimeter points.

2. add coord_sf() to convert the coordinates to a projection, in this case using "WGS 84 / World Equidistant Cylindrical"

3. remove the gray outline, default of which obscures most UK silhouette because its too thick.

---

library(dplyr); library(ggplot2)
data %>%
  left_join(world, by = c("country" = "region")) %>%
  ggplot(aes(x = long, y = lat, group = group)) +
  geom_polygon(aes(fill = value)) +
  coord_sf(crs = 4087) +
  scale_fill_gradient(low = "blue", high = "red") +
  theme_void()