Add a scale bar and a north arrow outside of the plot area of a facetted map plot

Question

I wish to add a scale bar and a north arrow outside of the plot area of a facetted map plot. As an example, consider the following facetted map plot, as seen in a blog post by Max Marchi (link):

# Load the data
airports <- read.csv("https://raw.githubusercontent.com/jpatokal/openflights/master/data/airports.dat", header = FALSE)
colnames(airports) <- c("ID", "name", "city",
  "country", "IATA_FAA", "ICAO", "lat", "lon",
   "altitude", "timezone", "DST")

routes <- read.csv("https://github.com/jpatokal/openflights/raw/master/data/routes.dat", header = FALSE)
colnames(routes) <- c("airline", "airlineID",
  "sourceAirport", "sourceAirportID",
  "destinationAirport", "destinationAirportID",
  "codeshare", "stops", "equipment")

# Getting the data ready for plotting
# * For a detailed explanation on setting up the data I suggest consulting Max Marchi's post: 
# http://www.milanor.net/blog/maps-in-r-plotting-data-points-on-a-map/
library(plyr)
departures <- ddply(routes, .(sourceAirportID), "nrow")
names(departures)[2] <- "flights"
arrivals <- ddply(routes, .(destinationAirportID), "nrow")
names(arrivals)[2] <- "flights"
airportD <- merge(airports, departures, by.x = "ID", 
                  by.y = "sourceAirportID")
airportA <- merge(airports, arrivals, by.x = "ID", 
                  by.y = "destinationAirportID")
airportD$type <- factor("departures")
airportA$type <- factor("arrivals")

# The final data frame used for plotting
airportDA <- rbind(airportD, airportA)

# Get the map of Europe from Google Maps
library(ggmap)
map <- get_map(location = 'Europe', zoom = 4)

# Make a facetted map plot 
library(ggplot2)
facet.gmap <- ggmap(map) +
  geom_point(aes(x = lon, y = lat, 
  size = sqrt(flights)), 
  data = airportDA, alpha = .5) +
  scale_size(breaks = sqrt(c(1, 5, 10, 50, 100, 500)), 
  labels = c(1, 5, 10, 50, 100, 500), name = "routes") + 
  theme(legend.position = "bottom",
        legend.direction="horizontal") +
  guides(size=guide_legend(nrow=1)) +  
  facet_wrap( ~ type, ncol=2)  
facet.gmap

I wish to add a scale bar and a north arrow outside of the plot area, at the right side of the plot legend for instance, to obtain something like this:

I tried using the scalebar and north functions of the ggsn package, however these functions only allow adding scale bars and north arrows inside of the plot area. My attempts using these functions are as follows:

# Adding a scale bar (but can't positioned it outside of the plot area)
library(ggsn)
bb <- attr(map, "bb")
bb2 <- data.frame(long = unlist(bb[c(2, 4)]), lat = unlist(bb[c(1,3)]))

scale.bar <- facet.gmap +
  scalebar(data = bb2, dist = 250, dd2km = TRUE, 
    model  = "WGS84",
    st.size=2.5,
    height=0.015,  
    location = "topleft",
    facet.var='airportDA$type',
    facet.lev='departures', 
    anchor = c(
      x = bb$ll.lon + 0.05 * (bb$ur.lon - bb$ll.lon), 
      y = bb$ll.lat + 0.9 * (bb$ur.lat - bb$ll.lat) )) 
scale.bar

# Adding a north arrow (but can't positioned it outside of the plot area)
north2(scale.bar,x=0.12,y=0.90, scale=0.09, symbol=3) 

Does anyone have any suggestion? Thanks in advance.

Answer 1

I actually figured it out! I know I'm like 7 months late to the show...

remove 'airport$' from facet.var, and it suddenly works.

scale.bar <- facet.gmap +
  scalebar(data = bb2, dist = 250, dd2km = TRUE, 
           model  = "WGS84",
           st.size=2.5,
           height=0.015,  
           location = "topleft",
           facet.var='type',
           facet.lev="arrivals", 
           anchor = c(
             x =-10, 
             y = 65 )) 
scale.bar

# Adding a north arrow (but can't positioned it outside of the plot area)
north2(scale.bar,x=0.82,y=0.17, scale=0.09, symbol=3)