Size legend of sf object won't show correct symbols

Question

Does anyone know why the legend of the size aestatic BIR74 won't show the dot sizes but rectangles? If the answer is yes, how can I fix this?

Reproducable example:

library(sf)
# devtools::install_github("tidyverse/ggplot2")
library(ggplot2)

nc <- st_read(system.file("shape/nc.shp", package="sf"))

nc_centers <- st_centroid(nc)

nc_centers %>%
  ggplot() +
  geom_sf(aes(color = SID79, size = BIR74)) +
  coord_sf(datum = NA) +
  theme_minimal()

Answer 1

you need to add the show.legend argument to geom_sf, i.e.

nc_centers %>%
  ggplot() +
  geom_sf(aes(color = SID79, size = BIR74), show.legend = 'point') +
  coord_sf(datum = NA) +
  theme_minimal()