feat(plot): add JGD graphics device integration

Embed the JGD (JSON Graphics Device) VS Code extension directly into
vscode-R so users only need the `jgd` R package for interactive,
Canvas2D-rendered plots with history, navigation, and PNG/SVG export.

Add `r.plot.backend` enum setting (auto/standard/httpgd/jgd) with
backward compatibility for existing `r.plot.useHttpgd` configurations.
The JGD socket server starts eagerly on activation when the backend is
set to "jgd", passing JGD_SOCKET to R terminals via env vars.

Closes #1679

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: grantmcdermott

R/profile.R

@@ -25,8 +25,10 @@ local({
 })
 
 if (requireNamespace("sess", quietly = TRUE)) {
+    plot_backend <- Sys.getenv("SESS_PLOT_BACKEND", "standard")
     sess::connect(
         use_rstudioapi = as.logical(Sys.getenv("SESS_RSTUDIOAPI", "TRUE")),
-        use_httpgd = as.logical(Sys.getenv("SESS_USE_HTTPGD", "TRUE"))
+        use_httpgd = (plot_backend == "httpgd"),
+        use_jgd = (plot_backend == "jgd")
     )
 }

package.json

@@ -1841,7 +1841,45 @@
         "r.plot.useHttpgd": {
           "type": "boolean",
           "default": false,
-          "markdownDescription": "Use the httpgd-based plot viewer instead of the base VSCode-R plot viewer.\n\nRequires the `httpgd` R package version 1.2.0 or later."
+          "markdownDescription": "Use the httpgd-based plot viewer instead of the base VSCode-R plot viewer.\n\nRequires the `httpgd` R package version 1.2.0 or later.",
+          "markdownDeprecationMessage": "Use `#r.plot.backend#` instead."
+        },
+        "r.plot.backend": {
+          "type": "string",
+          "default": "auto",
+          "enum": [
+            "auto",
+            "standard",
+            "httpgd",
+            "jgd"
+          ],
+          "markdownEnumDescriptions": [
+            "Automatic: uses httpgd if `#r.plot.useHttpgd#` is true, otherwise standard",
+            "Standard static plot viewer (PNG/SVG)",
+            "httpgd-based interactive plot viewer (requires `httpgd` R package)",
+            "JGD-based interactive plot viewer (requires `jgd` R package)"
+          ],
+          "markdownDescription": "Select the plot backend.\n\nWhen set to `auto`, the backend is determined by the legacy `#r.plot.useHttpgd#` setting."
+        },
+        "r.plot.jgd.historyLimit": {
+          "type": "number",
+          "default": 50,
+          "description": "Maximum number of plots retained in JGD history per session."
+        },
+        "r.plot.jgd.exportWidth": {
+          "type": "number",
+          "default": 7,
+          "description": "Default export width in inches for JGD plots."
+        },
+        "r.plot.jgd.exportHeight": {
+          "type": "number",
+          "default": 7,
+          "description": "Default export height in inches for JGD plots."
+        },
+        "r.plot.jgd.exportDpi": {
+          "type": "number",
+          "default": 150,
+          "description": "Default export DPI for JGD plots."
         },
         "r.plot.format": {
           "type": "string",

sess/DESCRIPTION

@@ -17,5 +17,6 @@ Imports:
     rstudioapi,
     svglite
 Suggests:
+    jgd,
     testthat (>= 3.0.0)
 Config/roxygen2/version: 8.0.0

sess/R/hooks.R

@@ -2,8 +2,9 @@
 #'
 #' @param use_rstudioapi Logical. Enable rstudioapi emulation.
 #' @param use_httpgd Logical. Enable httpgd plot device if available.
+#' @param use_jgd Logical. Enable jgd plot device if available.
 #' @export
-register_hooks <- function(use_rstudioapi = TRUE, use_httpgd = TRUE) {
+register_hooks <- function(use_rstudioapi = TRUE, use_httpgd = TRUE, use_jgd = FALSE) {
   # 1. Override View() to serve table data via paged RPC.
   show_dataview <- function(x, title = deparse(substitute(x))) {
     # make sure title is computed.
@@ -112,8 +113,12 @@ register_hooks <- function(use_rstudioapi = TRUE, use_httpgd = TRUE) {
     invisible(x)
   }
 
-  # 4. httpgd or Static Plot Hook
-  if (use_httpgd && requireNamespace("httpgd", quietly = TRUE)) {
+  # 4. Plot device: JGD > httpgd > Standard
+  if (use_jgd && nzchar(Sys.getenv("JGD_SOCKET")) && requireNamespace("jgd", quietly = TRUE)) {
+    options(device = function(...) {
+      jgd::jgd()
+    })
+  } else if (use_httpgd && requireNamespace("httpgd", quietly = TRUE)) {
     options(device = function(...) {
       httpgd::hgd(silent = TRUE)
       notify_client("httpgd", list(url = httpgd::hgd_url()))

sess/R/server.R

@@ -5,8 +5,9 @@
 #'   session JSON file written by the extension.
 #' @param use_rstudioapi Logical. Enable rstudioapi emulation. Defaults to TRUE.
 #' @param use_httpgd Logical. Use httpgd for plotting if available. Defaults to TRUE.
+#' @param use_jgd Logical. Use jgd for plotting if available. Defaults to FALSE.
 #' @export
-connect <- function(pipe_path = NULL, use_rstudioapi = TRUE, use_httpgd = TRUE) {
+connect <- function(pipe_path = NULL, use_rstudioapi = TRUE, use_httpgd = TRUE, use_jgd = FALSE) {
   .sess_env$con <- NULL
   .sess_env$pending_responses <- list()
   .sess_env$read_buffer <- ""
@@ -90,7 +91,8 @@ connect <- function(pipe_path = NULL, use_rstudioapi = TRUE, use_httpgd = TRUE)
 
   if (is.na(use_rstudioapi)) use_rstudioapi <- TRUE
   if (is.na(use_httpgd)) use_httpgd <- TRUE
-  register_hooks(use_rstudioapi = use_rstudioapi, use_httpgd = use_httpgd)
+  if (is.na(use_jgd)) use_jgd <- FALSE
+  register_hooks(use_rstudioapi = use_rstudioapi, use_httpgd = use_httpgd, use_jgd = use_jgd)
 
   invisible(NULL)
 }