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I have the following code

ggplot(data=earthquake,aes(longitude,latitude))+
  geom_point(data=earthquake,aes(longitude,latitude),alpha=0.5,col="red",fill="red",shape=21)+
  coord_map(projection = "mollweide",xlim=c(-180,180))+
  xlab("Galactic Longitude")+
  ylab("Galactic Latitude")+
  scale_x_continuous(breaks=seq(-180,180,60),limits=c(-180,180))+
  scale_y_continuous(breaks=seq(-90,90,30))

where earthquake is a matrix of columns "longitude" and "latitude" between -180 and 180, and -90 and 90, respectively. It produces the following plot and clearly the x-axis ticks aren't spaced as they should be.

I would ideally like to recreate this plot in ggplot2 with nicely spaced ticks, and hopefully the ablines of longitude/latitude.

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  • 2
    I don't think you can put the axis inside the plot without turning to other packages, but the ggh4x package might let you do it as here. Or you could remove the x-axis and plot a line and label it with geom_text(). Commented Jun 26, 2025 at 19:42

1 Answer 1

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One option could be to manually create the longitude / latitude labels with geom_text with coord_sf(crs = moll_crs, default_crs = st_crs(4326), datum = NA) at the end as pointed out by @IO. You can use hjust / vjust to change the label alignments as per this and size to controll the text size. out

library(sf)
library(ggplot2)

moll_crs <- "+proj=moll +lon_0=0 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs"

earthquake_sf <- st_as_sf(geostats::earthquakes, coords = c("lon", "lat"), crs = 4326) |>
  st_transform(crs = moll_crs)

graticule <- st_graticule(lat = seq(-90, 90, 30), lon = seq(-180, 180, 60), crs = 4326) |>
  st_transform(moll_crs)

ggplot() +
  geom_sf(data = graticule, color = "black", size = 0.3, alpha = 0.7) +
  geom_sf(data = earthquake_sf, color = "black", alpha = 0.5, size = 1) +
  # place latitude labels 
  geom_text(aes(x = -180, y = seq(-60, 60, 30), 
                label = paste0(seq(-60, 60, 30), "°")), 
            size = 5, hjust = 1.5) +
  # place longitude labels
  geom_text(aes(x = seq(-120, 120, 60), y = 0,  
                label = paste0(seq(-120, 120, 60), "°")), 
            size = 5, vjust = 1.5, hjust = 1) +
  # place 90 on top / below
  geom_text(aes(x = -180, y = 90, label = "90°"), 
            size = 5, vjust = -0.5) +
  geom_text(aes(x = -180, y = -90, label = "-90°"), 
            size = 5, vjust = 1.5) +
  coord_sf(crs = moll_crs, default_crs = st_crs(4326), datum = NA) +
  theme_minimal() + 
  labs(x = expression(alpha[2000])) +
  theme(axis.title.y = element_blank(), axis.title.x=element_text(size=20))

Note: coord_sf() is designed specifically to work with geom_sf() and spatial data that's already in a proper coordinate reference system (CRS). When you try to use coord_sf() with regular geoms like geom_point() you run the risk of causing several problems like difficult to place gratitude labels.

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2 Comments

It might be useful to be more up-front with the fact that coord_sf() does not work well with non-sf geom_*. It may be obvious to many, but it may not be intuitive to everybody.
Hint: using ... + coord_sf(default_crs = st_crs(4326), datum = NA, ...) allows to (1) use vanilla geom_text (with aesthetics x and y given in degrees) and (2) hide the default graticule.

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