Drawing arbitrary lines on a highchart map in R (library highcharter) - r

Question

I am drawing a highcharts map using the highcharter package in R. I added already some points (cities) and want to link them by drawing an additionnal beeline using the world map-coordinates.
I already managed to draw the beelines by first drawing the map, then hovering over the cities which shows me the plot-coordinates, and then redrawing the plot using the aforementioned plot-coordinates. (Watch out: I used the PLOT-coordinates and my goal is to use directly the WORLD MAP-coordinates.)
If you only have 1 or two cities, it's not a big deal. But if you have like 100 cities/points, it's annoying. I guess the answer will be something like here: Is it possible to include maplines in highcharter maps?.
Thank you!
Here my code:
library(highcharter)
library(tidyverse)
# cities with world coordinates
ca_cities <- data.frame(
name = c("San Diego", "Los Angeles", "San Francisco"),
lat = c(32.715736, 34.052235, 37.773972), # world-map-coordinates
lon = c(-117.161087, -118.243683, -122.431297) # world-map-coordinates
)
# path which I create AFTER the first drawing of the map as I get the
# plot-coordinates when I hover over the cities.
path <- "M669.63,-4963.70,4577.18,-709.5,5664.42,791.88"
# The goal: the path variable above should be defined using the WORLD-
# coordinates in ca_cities and not using the PLOT-coordinates.
# information for drawing the beeline
ca_lines <- data.frame(
name = "line",
path = path,
lineWidth = 2
)
# construct the map
map <- hcmap("countries/us/us-ca-all", showInLegend = FALSE) %>%
hc_add_series(data = ca_cities, type = "mappoint", name = "Cities") %>%
hc_add_series(data = ca_lines, type = "mapline", name = "Beeline", color = "blue")
map
See picture here

Answer 1

After several hours, I found an answer to my problem. There are maybe easier ways, but I'm going to post my version using the rgdal-package.
The idea is to convert first the world map-coordinates to the specific map's coordinate system (ESRI) and then back-transform all adjustments from highcharts:
library(highcharter)
library(tidyverse)
library(rgdal) # you also need rgdal
# cities with world coordinates
ca_cities <- data.frame(
name = c("San Diego", "Los Angeles", "San Francisco"),
lat = c(32.715736, 34.052235, 37.773972),
lon = c(-117.161087, -118.243683, -122.431297)
)
# pre-construct the map
map <- hcmap("countries/us/us-ca-all", showInLegend = FALSE)
# extract the transformation-info
trafo <- map$x$hc_opts$series[[1]]$mapData$`hc-transform`$default
# convert to coordinates
ca_cities2 <- ca_cities %>% select("lat", "lon")
coordinates(ca_cities2) <- c("lon", "lat")
# convert world geosystem WGS 84 into transformed crs
proj4string(ca_cities2) <- CRS("+init=epsg:4326") # WGS 84
ca_cities3 <- spTransform(ca_cities2, CRS(trafo$crs)) #
# re-transform coordinates according to the additionnal highcharts-parameters
image_coords_x <- (ca_cities3$lon - trafo$xoffset) * trafo$scale * trafo$jsonres + trafo$jsonmarginX
image_coords_y <- -((ca_cities3$lat - trafo$yoffset) * trafo$scale * trafo$jsonres + trafo$jsonmarginY)
# construct the path
path <- paste("M",
paste0(paste(image_coords_x, ",", sep = ""),
image_coords_y, collapse = ","),
sep = "")
# information for drawing the beeline
ca_lines <- data.frame(
name = "line",
path = path,
lineWidth = 2
)
# add series
map <- map %>%
hc_add_series(data = ca_cities, type = "mappoint", name = "Cities") %>%
hc_add_series(data = ca_lines, type = "mapline", name = "Beeline", color = "blue")
map