Preserve axis labels in geom_sf when changing projection

Question

I'm trying to show axis labels on a world map using Robinson projection. I have no problem showing the labels in default projection:

library(tidyverse)
library(sf)
library(rnaturalearth)

world <- ne_countries(scale = "small", returnclass = "sf")

world %>%
  ggplot() +
  geom_sf() +
  coord_sf(label_axes = "E--N",
           expand = FALSE)

but when I try

world %>%
  ggplot() +
  geom_sf() +
  coord_sf(label_axes = "E--N",
           expand = FALSE,
           crs = "+proj=robin")

the y axis labels are messed up. Any suggestions how to fix/hack this? Ideally I get something like (but better formatted).

Answer 1

I think you'll need to do this manually:

grid <- list(
       st_sf(geometry = st_sfc(
  st_multilinestring(x = lapply(c(-180, -120, -60, 0, 60, 120, 180), 
                function(x) cbind(x, seq(-90, 90, 1)))), crs = 'WGS84')),
        st_sf(geometry = st_sfc(
  st_multilinestring(x = lapply(c(-90, -60, -30, 0, 30, 60, 90), function(x) {
    cbind(seq(-180, 180, 1), x)
  })), crs = 'WGS84'))) %>% bind_rows()

ylabs <- lapply(c(-90, -60, -30, 0, 30, 60, 90), function(x) {
  st_sf(label = paste0(abs(x), '\u00b0', 
                       ifelse(x == 0, '', ifelse(x < 0, 'S', 'N'))),
        geometry = st_sfc(st_point(c(-180, x)), crs = 'WGS84'))
}) %>% bind_rows()

world %>%
  ggplot() +
  geom_sf(fill = 'white') +
  geom_sf(data = grid, linewidth = 0.1) +
  geom_sf_text(data = ylabs, aes(label = label), size = 3, color = 'gray30',
               nudge_x = c(-2e6, -1.5e6, -1.3e6, -1e6, -1.3e6, -1.5e6, -2e6)) +
  coord_sf(label_axes = "E--N",
           expand = FALSE,
           crs = "+proj=robin",
           clip = "off") +
  theme_minimal() +
  theme(axis.text.y = element_blank(),
        panel.grid = element_blank(),
        axis.title = element_blank(),
        plot.margin = margin(10, 10, 10, 30))