Plot points outside grid as arrows pointing to data with ggplot2 in R

Question

I am generating maps with world-scale data, and then zooming in to certain regions. On the zoomed-in view, I would like to show that there are other data points outside the bounding box, by putting arrowheads that point from the center of the box to where the data point is in the outside world.

Note: I do not need it to be a "great circle" path, just XY vectors in Mercator projection, because I imagine this will be useful for "normal" plots as well.

As an example, here is the world map showing the extent of the data:

And here is the zoomed in view, with magenta arrows manually added to show what I would like to generate.

Below is the code and data I am using to generate these two basic plots. What I need is a way to generate the arrowheads.

require(ggplot2)

te = structure(list(lat = c(33.7399, 32.8571, 50.2214, 36.96263, 33.5835, 
33.54557, 47.76147, 48, 59.40289, 35.93411, 32.87962, 38.3241, 
50.03844, 37.44, 50.07774, 50.26668, 36.5944), lng = c(-118.37608, 
-117.25746, -5.3865, -122.00809, -117.86159, -117.79805, -124.45055, 
-126, -146.35157, -122.931472, -117.25285, -123.07331, -5.26339, 
25.4, -5.709894, -3.86828, -121.96201)), .Names = c("lat", "lng"
), class = "data.frame", row.names = c(NA, -17L))

all_states = map_data("world")

# world version:
wp = ggplot() + 
      geom_polygon(data = all_states, aes(x = long, y = lat, group = group), colour = "gray",
                   fill = "gray") +
      coord_cartesian(ylim = c(0, 80), xlim = c(-155, 45)) + 
      geom_point(data = te, aes(x = lng, y = lat), color = "blue", size = 5,alpha = 0.6)

print(wp)

#states plot
sp = ggplot() +
      geom_polygon(data = all_states, aes(x = long, y = lat, group = group), colour = "gray", fill = "gray") +
      coord_cartesian(ylim = c(30, 52), xlim = c(-128, -114)) + 
      geom_point(data = te, aes(x = lng, y = lat), color = "blue", size = 5, alpha = 0.6) 

print(sp)

Answer 1

This solution uses sp and rgeos packages to manipulate spatial data, the main crux being intersecting lines and a box polygon to get the edge points for arrows. Then if you draw arrows with geom_segment and zero width, the line is invisible and only the arrow head remains.

This function computes the line-box intersections:

boxint <- function(xlim, ylim, xp, yp){
    ## build box as SpatialPolygons
    box = cbind(xlim[c(1,2,2,1,1)],
        ylim[c(1,1,2,2,1)])
    box <- sp::SpatialPolygons(list(sp::Polygons(list(sp::Polygon(box)),ID=1)))

    ## get centre of box
    x0=mean(xlim)
    y0=mean(ylim)

    ## construct line segments to points
    sl = sp::SpatialLines(
        lapply(1:length(xp),
               function(i){
                   sp::Lines(list(sp::Line(cbind(c(x0,xp[i]),c(y0,yp[i])))),ID=i)
               }
               )
        )
    ## intersect lines segments with boxes to make points
    pts = rgeos::gIntersection(sl, as(box, "SpatialLines"))
    as.data.frame(sp::coordinates(pts), row.names=1:length(xp))
}

And this returns the geom with arrows:

wherelse <- function(xlim, ylim, points){
    ## get points outside bounding box
    outsides = points[!(
        points$lng>=xlim[1] &
            points$lng <= xlim[2] &
                points$lat >= ylim[1] &
                    points$lat <= ylim[2]),]
    npts = nrow(outsides)
    ## get centre point of box
    x = rep(mean(xlim),npts)
    y = rep(mean(ylim),npts)

    ## compute box-point intersections
    pts = boxint(xlim, ylim, outsides$lng, outsides$lat)
    pts$x0=x
    pts$y0=y
    ## create arrow segments as invisible lines with visible arrowheads
    ggplot2::geom_segment(data=pts, aes(x=x0,y=y0,xend=x,yend=y),
       lwd=0, arrow=grid::arrow(length=unit(0.5,"cm"),
       type="closed"),col="magenta")
}

So your example, the basic plot is:

sp = ggplot() + 
  geom_polygon(
   data=all_states, 
    aes(x=long, y=lat, group = group),colour="gray",fill="gray" ) + 
    coord_cartesian(ylim=c(30, 52), xlim=c(-128,-114)) + 
    geom_point(data=te,aes(x=lng,y=lat),color="blue",size=5,alpha=0.6)

and then add the arrows with:

sp + wherelse(c(-128,-114), c(30,52), te)

Not sure if there's an option to draw arrows exactly like you want them though!

Answer 2

Here is my attempt. This is the closest I got. I used gcIntermediate() for calculating the shortest distance between the center point of your US map and the data points which stay outside of the bbox. Hence, the arrow positions may not be something you want. My hope is that somebody else would deliver a better solution based on this attempt.

I first arranged your df (i.e., te) with the center point in the US zoomed map. I then chose data points which are not in the bbox of the US map. Then, add two columns to indicate the center point of the US map. Rename two columns and calculate the shortest distance with gcIntermediate.

library(dplyr)
library(ggplot2)
library(geosphere)

filter(te, !between(lng, -128, -114) | !between(lat, 30, 52)) %>%
mutate(start_long = (-128 - 114) / 2,
       start_lat = (30 + 52) / 2) %>%
rename(end_lat = lat, end_long = lng) %>%
do(fortify(as(gcIntermediate(.[,c("start_long", "start_lat")],
                             .[,c("end_long", "end_lat")],
                             100,
                             breakAtDateLine = FALSE,
                             addStartEnd = TRUE,
                             sp = TRUE), "SpatialLinesDataFrame"))) -> foo

foo contains 100 data points to draw respective line. I chose data points which stay close to the bbox boundary. I was specifically looking for two data points for each line so that I could use geom_segment() later. I admit that I played with the filter condition a bit. In the end, I did not subset data using lat in this case.

filter(foo, between(long, -128, -126.5) | between(long, -115.5, -114)) %>%
group_by(group) %>%
slice(c(1,n())) -> mydf

In the next step, I rearranged the data frame based on this link

mutate(mydf, end_long = lag(long), end_lat = lag(lat)) %>%
slice(n()) -> mydf2

Finally I drew the map with arrows. I hope this will provide some kind of base for you. I also hope that other SO users will provide better solutions.

ggplot() +
geom_polygon(data = all_states, aes(x = long, y = lat, group = group),
             colour = "gray", fill = "gray" ) +
coord_cartesian(ylim = c(30, 52), xlim = c(-128,-114)) +
geom_point(data = te, aes(x = lng,y = lat), color = "blue", size = 5,alpha = 0.6) +
geom_segment(data = mydf2, aes(x = end_long, xend = long,
                               y = end_lat, yend = lat, group = group),
                               arrow = arrow(length = unit(0.2, "cm"), ends = "last"))