geom_rect + coord_map = very slow

Question

I am plotting a map of Norway with an area of interest highlighted with a red rectangle using ggplot2. If I omit the geom_rect or coord_map, the map plots very quickly (< 1 seconds). If I use both - which I need to - it is extremely slow to print and render (about five minutes).

I presume this is something to do with the munching - projecting the rectangle onto the new coordinate system. Is there a way to control this?

library(ggplot2)
library(maps)
library(mapdata)


xlim <- c(5, 10)
ylim <- c(60, 62)

norwaymap <- map_data("worldHires", "Norway")
a <- ggplot(norwaymap, aes(x = long, y = lat, group = group)) +
  geom_polygon(colour = NA, fill = "grey60") +
  geom_rect(xmin = xlim[1], xmax = xlim[2], ymin = ylim[1], ymax = ylim[2], 
    colour = "red", fill = NA) +
  coord_map(xlim = c(3, 33), ylim = c(57, 72))
print(a) # super slow

Using the low resolution map makes the map plotting much faster (about 10 seconds).

Answer 1

No need to resort to mercator approximations:

library(ggplot2)
library(maps)
library(mapdata)

norwaymap <- map_data("worldHires", "Norway")

xlim <- c(5, 10)
ylim <- c(60, 62)

ggplot() +
  geom_map(data=norwaymap, map=norwaymap,
           aes(long, lat, map_id=region),
           color=NA, fill="grey60") +
  geom_rect(data=data.frame(),
            aes(xmin=xlim[1], xmax=xlim[2], ymin=ylim[1], ymax=ylim[2]),
            color="red", fill=NA) +
  coord_map(xlim=c(3, 33), ylim=c(57, 72)) +
  ggthemes::theme_map()

Another option would be to use an Albers equal-area conic projection (a typical one for that region):

ggplot() +
  geom_map(data=norwaymap, map=norwaymap,
           aes(long, lat, map_id=region),
           color=NA, fill="grey60") +
  geom_rect(data=data.frame(),
            aes(xmin=xlim[1], xmax=xlim[2], ymin=ylim[1], ymax=ylim[2]),
            color="red", fill=NA) +
  ggalt::coord_proj("+proj=aea +lat_1=60 +lat_2=70 +lon_0=18.37", 
                    xlim=c(3, 33), ylim=c(57, 72)) +
  ggthemes::theme_map()

That has a "disadvantage" of the rectangle being projected (it is with Mercator, too, there's just no distortion).

Either way, the magic for the rectangle is ensuring you're plotting only one, like Luke said.

Answer 2

Use coord_quickmap and especially annotate instead of geom_rect to speed things up:

ggplot(norwaymap, aes(x = long, y = lat, group = group)) +
  geom_polygon(colour = NA, fill = "grey60") +
  annotate(geom="rect", xmin = xlim[1], xmax = xlim[2], ymin = ylim[1], 
           ymax = ylim[2], colour = "red", fill = NA) + 
  coord_quickmap(xlim = c(3, 33), ylim = c(57, 72))

geom_rect overplots several rectangles on the same spot, annotate just plots one rectangle. You can read about the difference between coord_map and coord_quickmap in the help files: ?coord_quickmap.