ggplot create map with arrows - r

Question

I have a data frame like this
id lon lat
1 A -69.5 -58.5
2 A -69.5 -58.5
3 A -69.5 -57.5
4 A -68.5 -57.5
5 A -68.5 -57.5
6 A -68.5 -57.5
7 A -66.5 -57.5
8 A -68.5 -56.5
9 A -68.5 -56.5
10 A -67.5 -56.5
11 A -65.5 -56.5
12 A -65.5 -56.5
13 A -65.5 -55.5
14 A -62.5 -54.5
15 B -177 -52.5
16 B -178 -50.5
17 B -179 -48.5
18 B 179 -47.5
19 B 178 -46.5
20 B 177 -46.5
and I want to produce a map of the position of A and B, linked by oriented lines. However when ids cross the Pacific (lon=-180 -> lon=+180) I get an arrow crossing the whole figure, like shown below.
This is the code I am using
worldmap = map_data("world")
ggplot(test, aes(x = lon, y=lat, colour = factor(id))) +
geom_polygon(data=worldmap,center=180,aes(x=long, y=lat, group=group), fill="black",colour="black") +
xlab("") +ylab("")+theme(axis.text=element_blank(),axis.ticks=element_blank())+ theme(panel.background = element_rect(fill = 'white', colour = 'black') ,panel.grid.major = element_blank(),panel.grid.minor = element_blank())+
geom_path(size =2,arrow = arrow(angle=30,length = unit(0.6, "inches")))
How can I fix it?
Thanks

Answer 1

I guess that depends on what you think the "right" think to do is. I decided to break up the pathes that cross the glob into two segments by adding in points at the edge of the map, and then creating a "sequence" indicator so ggplot knows which lines to connect. Here's the transformation for your sample data
test2 <- do.call(rbind, lapply(split(test, test$id), function(x) {
cp <- cumsum(c(FALSE, diff(x$lon)>250))
xx<-split(x, cp)
xx<-Map(cbind, xx, seq=seq_along(xx))
Reduce(function(a,b) {
lasta<-a[nrow(a),]
firstb<-b[1,]
lasta$lon <- 180*sign(lasta$lon)
firstb$lon <- 180*sign(firstb$lon)
lasta$lat <- mean(lasta$lat, firstb$lat)
firstb$lat <- lasta$lat
rbind(a,lasta, firstb,b)
}, xx)
}))
tail(test2)
# id lon lat seq
# B.17 B -179 -48.5 1
# B.171 B -180 -48.5 1
# B.18 B 180 -48.5 2
# B.181 B 179 -47.5 2
# B.19 B 178 -46.5 2
# B.20 B 177 -46.5 2
here you can see that we've broken the B line up into two sequences. Then if we use a group aesthetic
geom_path(aes(group=interaction(id, seq)), ...)
then R will only connect those points that are in the same id/seq group. This will prevent the line from going across the ocean. However, because we are drawing two lines for that group rather than one, there's no way to turn of the arrow head for just one of the segments. you might want to find another way to indicate start/end.