add offline argument to plot_geo(); closes #356

Author: cpsievert

Diff for: DESCRIPTION

@@ -67,7 +67,8 @@ Suggests:
     rgeos,
     png,
     IRdisplay,
-    processx
+    processx,
+    plotlyGeoAssets
 Remotes:
   tidyverse/ggplot2
 LazyData: true

Diff for: R/plotly.R

@@ -253,6 +253,9 @@ plot_mapbox <- function(data = data.frame(), ...) {
 #' 
 #' @param data A data frame (optional).
 #' @param ... arguments passed along to [plot_ly()].
+#' @param offline whether or not to include geo assets so that the map
+#' can be viewed with or without an internet connection. The plotlyGeoAssets
+#' package is required for this functionality.
 #' @export
 #' @author Carson Sievert
 #' @seealso [plot_ly()], [plot_mapbox()], [ggplotly()] 
@@ -263,8 +266,22 @@ plot_mapbox <- function(data = data.frame(), ...) {
 #'   plot_geo(x = ~long, y = ~lat) %>%
 #'   add_markers(size = I(1))
 #' 
-plot_geo <- function(data = data.frame(), ...) {
+plot_geo <- function(data = data.frame(), ..., offline = FALSE) {
   p <- plot_ly(data, ...)
+  
+  if (isTRUE(offline)) {
+    if (system.file(package = "plotlyGeoAssets") == "") {
+      stop(
+        "The plotlyGeoAssets package is required to make 'offline' maps. ",
+        "Please install and try again.",
+        call. = FALSE
+      )
+    }
+    p$dependencies <- c(
+      list(plotlyGeoAssets::geo_assets()),
+      p$dependencies
+    )
+  }
   # not only do we use this for is_geo(), but also setting the layout attr
   # https://plot.ly/r/reference/#layout-geo
   p$x$layout$mapType <- "geo"