Create ggmap with points, faceted, and each facet zoomed appropriately? - r

Question

I want to plot data points for various cities over a greyed-out map from google. As these cities are some distance from each other, I thought I would use a faceted plot.
Creating the map is easy enough; see image and code below. However, each facet shows the same area - in this case Greater London - with the result that the points for other cities are not shown.
Ideally I would like each facet to show each city with the relevant points overlaid. So the facet 'Cardiff' would show a zoomed map of Cardiff and its data points, 'Birmingham' would show Birmingham and its points and so on. I've tried changing various parameters such as zoom and center but I haven't been successful.
How can I show a different city and the relevant points in each facet?
require(ggmap)
require(reshape)
# create fake data
sites <- data.frame(site = 1:6,
name = c(
"Royal Albert Hall",
"Tower of London",
"Wales Millenium Centre",
"Cardiff Bay Barrage",
"Birmingham Bullring",
"Birmingham New Street Station"
),
coords = c(
"51.501076,-0.177265",
"51.508075,-0.07605",
"51.465211,-3.163208",
"51.44609,-3.166652",
"52.477644,-1.894158",
"52.477487,-1.898836"),
subzone = rep(c('London','Cardiff','Birmingham'), each = 2)
)
# use function from reshape to split/add column
sites = transform(sites,
new = colsplit(coords, split = ",", names = c('lat', 'lon')))
names(sites) <- c(names(sites)[1:4], 'lat','lon')
ggmap(get_googlemap(center = "London", # omitting this doesn't help
scale = 2,
zoom = 11, # fiddling with zoom doesn't work
color = 'bw',
maptype = 'roadmap',
extent = 'panel',
format = "png8",
filename = "facet_map_test",
)) +
facet_wrap(~ subzone, ncol = 1) +
geom_point(data = sites,
aes(x = lon, y = lat),
fill = "red",
size = 3,
colour = "black",
shape = 21,
alpha = 1) +
theme(legend.position = "none") +
theme()

Answer 1

Using the gridExtra package is probably the best way to go. The code:
# getting the maps
londonmap <- get_map(location = c(lon = -0.1266575, lat = 51.504575), zoom = 12)
cardiffmap <- get_map(location = c(lon = -3.16493, lat = 51.45565), zoom = 13)
birminghammap <- get_map(location = c(lon = -1.896497, lat = 52.477565), zoom = 14)
# plotting the maps
p1 <- ggmap(londonmap) +
geom_point(data = sites[sites$subzone == "London",],
aes(x = lon, y = lat, fill = "red", alpha = 0.8, size = 3),
shape = 21) +
ggtitle("London") +
theme(axis.title = element_blank(), legend.position = "none", plot.margin = unit(c(0,0,0,0), "lines"))
p2 <- ggmap(cardiffmap) +
geom_point(data = sites[sites$subzone == "Cardiff",],
aes(x = lon, y = lat, fill = "red", alpha = 0.8, size = 3),
shape = 21) +
ggtitle("Cardiff") +
theme(axis.title = element_blank(), legend.position = "none", plot.margin = unit(c(0,0,0,0), "lines"))
p3 <- ggmap(birminghammap) +
geom_point(data = sites[sites$subzone == "Birmingham",],
aes(x = lon, y = lat, fill = "red", alpha = 0.8, size = 3),
shape = 21) +
ggtitle("Birmingham") +
theme(axis.title = element_blank(), legend.position = "none", plot.margin = unit(c(0,0,0,0), "lines"))
# grouping the plots together in one plot
grid.arrange(p1, p2, p3, ncol = 1)
The result: