Lines between maps on multiple-map layout

gistfile1.txt

## Main code based on https://www.r-spatial.org//r/2018/10/25/ggplot2-sf-3.html

library("ggplot2")
theme_set(theme_bw())
## Requires up to date gdal
## (e.g., from https://launchpad.net/~ubuntugis/+archive/ubuntu/ppa)
library("sf")

library("rworldmap")
library("rworldxtra")
world <- getMap(resolution = "high")
## Requires libgeos++dev (not [just] libgeos-dev)
library("rgeos")
world <- st_as_sf(world)

sites <- st_as_sf(data.frame(longitude = c(-80.15, -80.1), latitude = c(26.5, 
    26.8)), coords = c("longitude", "latitude"), crs = 4326, 
    agr = "constant")

(florida <- ggplot(data = world) +
     geom_sf(fill = "antiquewhite1") +
     geom_sf(data = sites, size = 4, shape = 23, fill = "darkred") +
     annotate(geom = "text", x = -85.5, y = 27.5, label = "Gulf of Mexico", 
         color = "grey22", size = 4.5) +
     coord_sf(xlim = c(-87.35, -79.5), ylim = c(24.1, 30.8)) +
     xlab("Longitude")+ ylab("Latitude")+
     theme(panel.grid.major = element_line(colour = gray(0.5), linetype = "dashed", 
         size = 0.5), panel.background = element_rect(fill = "aliceblue"), 
         panel.border = element_rect(fill = NA)))

(siteA <- ggplot(data = world) +
     geom_sf(fill = "antiquewhite1") +
     geom_sf(data = sites, size = 4, shape = 23, fill = "darkred") +
     coord_sf(xlim = c(-80.25, -79.95), ylim = c(26.65, 26.95), expand = FALSE) + 
     annotate("text", x = -80.18, y = 26.92, label= "Site A", size = 6) + 
     theme_void() + 
     theme(panel.grid.major = element_line(colour = gray(0.5), linetype = "dashed", 
         size = 0.5), panel.background = element_rect(fill = "aliceblue"), 
         panel.border = element_rect(fill = NA)))

(siteB <- ggplot(data = world) + 
     geom_sf(fill = "antiquewhite1") +
     geom_sf(data = sites, size = 4, shape = 23, fill = "darkred") +
     coord_sf(xlim = c(-80.3, -80), ylim = c(26.35, 26.65), expand = FALSE) +
     annotate("text", x = -80.23, y = 26.62, label= "Site B", size = 6) + 
     theme_void() +
     theme(panel.grid.major = element_line(colour = gray(0.5), linetype = "dashed", 
         size = 0.5), panel.background = element_rect(fill = "aliceblue"), 
         panel.border = element_rect(fill = NA)))

arrowA <- data.frame(x1 = 18.5, x2 = 23, y1 = 9.5, y2 = 14.5)
arrowB <- data.frame(x1 = 18.5, x2 = 23, y1 = 8.5, y2 = 6.5)

ggplot() +
    coord_equal(xlim = c(0, 28), ylim = c(0, 20), expand = FALSE) +
    annotation_custom(ggplotGrob(florida), xmin = 0, xmax = 20, ymin = 0, 
        ymax = 20) +
    annotation_custom(ggplotGrob(siteA), xmin = 20, xmax = 28, ymin = 11.25, 
        ymax = 19) +
    annotation_custom(ggplotGrob(siteB), xmin = 20, xmax = 28, ymin = 2.5, 
        ymax = 10.25) +
    ## I'll draw the segments myself below
    ## geom_segment(aes(x = x1, y = y1, xend = x2, yend = y2), data = arrowA, 
    ##              arrow = arrow(), lineend = "round") +
    ## geom_segment(aes(x = x1, y = y1, xend = x2, yend = y2), data = arrowB, 
    ##              arrow = arrow(), lineend = "round") +
    theme_void()

## Custom line segment stuff starts here
library(grid)
## grid.ls()
grid.force(redraw=FALSE)
## grid.ls()
points <- grid.grep("points", grep=TRUE, global=TRUE, viewports=TRUE)
# Site A
depth <- downViewport(attr(points[[2]], "vpPath"))
p2 <- grid.get(points[[2]])
grid.move.to(grobX(p2, 0) + unit(2, "mm"), grobY(p2, 0) + unit(2, "mm"))
upViewport(depth)
depth <- downViewport(attr(points[[4]], "vpPath"))
p4 <- grid.get(points[[4]])
cvp <- current.viewport()
pushViewport(viewport(xscale=cvp$xscale, yscale=cvp$yscale, clip="off"))
grid.line.to(grobX(p4, 180) - unit(2, "mm"), grobY(p4, 180) - unit(2, "mm"),
             arrow=arrow())
upViewport(depth + 1)
## Site B
depth <- downViewport(attr(points[[1]], "vpPath"))
p1 <- grid.get(points[[1]])
grid.move.to(grobX(p1, 0) + unit(2, "mm"), grobY(p1, 0) - unit(1, "mm"))
upViewport(depth)
depth <- downViewport(attr(points[[5]], "vpPath"))
p5 <- grid.get(points[[5]])
cvp <- current.viewport()
pushViewport(viewport(xscale=cvp$xscale, yscale=cvp$yscale, clip="off"))
grid.line.to(grobX(p5, 180) - unit(2, "mm"), grobY(p5, 180) + unit(1, "mm"),
             arrow=arrow())
upViewport(depth + 1)