Fix #2407, #2187: legend.position theme element now translated to plotly layout

The legend.position theme element is now properly translated to plotly's
legend positioning:
- "bottom" -> horizontal orientation, positioned below plot
- "top" -> horizontal orientation, positioned above plot
- "left" -> positioned to the left of plot
- "right" -> default (no change)
- "none" -> legend hidden (was already working)
- "inside" / numeric c(x, y) -> custom position inside plot area

Supports both older ggplot2 (direct numeric vector) and newer ggplot2 >= 3.5.0
(legend.position = "inside" with legend.position.inside for coordinates).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: cpsievert

R/ggplotly.R

@@ -979,12 +979,47 @@ gg2list <- function(p, width = NULL, height = NULL,
     bgcolor = toRGB(theme$legend.background$fill),
     bordercolor = toRGB(theme$legend.background$colour),
     borderwidth = unitConvert(
-      theme$legend.background[[linewidth_or_size(theme$legend.background)]], 
+      theme$legend.background[[linewidth_or_size(theme$legend.background)]],
       "pixels", "width"
     ),
     font = text2font(theme$legend.text)
   )
-  
+
+  # Translate legend.position from ggplot2 theme to plotly layout (fixes #2407, #2187)
+  legend_pos <- theme$legend.position %||% theme[["legend.position"]]
+  if (!is.null(legend_pos) && !identical(legend_pos, "none")) {
+    if (is.character(legend_pos)) {
+      gglayout$legend <- switch(legend_pos,
+        "bottom" = modifyList(gglayout$legend, list(
+          orientation = "h", x = 0.5, y = -0.15, xanchor = "center", yanchor = "top"
+        )),
+        "top" = modifyList(gglayout$legend, list(
+          orientation = "h", x = 0.5, y = 1.02, xanchor = "center", yanchor = "bottom"
+        )),
+        "left" = modifyList(gglayout$legend, list(
+          x = -0.15, y = 0.5, xanchor = "right", yanchor = "middle"
+        )),
+        "inside" = {
+          # In ggplot2 >= 3.5.0, numeric position is stored in legend.position.inside
+          inside_pos <- theme$legend.position.inside %||% theme[["legend.position.inside"]]
+          if (is.numeric(inside_pos) && length(inside_pos) == 2) {
+            modifyList(gglayout$legend, list(
+              x = inside_pos[1], y = inside_pos[2], xanchor = "left", yanchor = "bottom"
+            ))
+          } else {
+            gglayout$legend
+          }
+        },
+        gglayout$legend  # "right" is default, no change needed
+      )
+    } else if (is.numeric(legend_pos) && length(legend_pos) == 2) {
+      # Handle numeric position like c(0.8, 0.2) for older ggplot2 versions
+      gglayout$legend <- modifyList(gglayout$legend, list(
+        x = legend_pos[1], y = legend_pos[2], xanchor = "left", yanchor = "bottom"
+      ))
+    }
+  }
+
   # if theme(legend.position = "none") is used, don't show a legend _or_ guide
   if (npscales$n() == 0 || identical(theme$legend.position, "none")) {
     gglayout$showlegend <- FALSE

tests/testthat/test-ggplot-theme.R

@@ -90,3 +90,45 @@ test_that("element_blank panel.border does not create empty shapes (#2455, #2460
   expect_true(length(bw_shapes) > 0)
   expect_false(is.na(bw_shapes[[1]]$line$color))
 })
+
+test_that("legend.position is translated correctly (#2407, #2187)", {
+  # Test legend.position = "bottom"
+  p_bottom <- ggplot(mtcars, aes(wt, mpg, color = factor(cyl))) +
+    geom_point() +
+    theme(legend.position = "bottom")
+  L_bottom <- plotly_build(ggplotly(p_bottom))$x
+  expect_equal(L_bottom$layout$legend$orientation, "h")
+  expect_equal(L_bottom$layout$legend$xanchor, "center")
+  expect_equal(L_bottom$layout$legend$y, -0.15)
+
+  # Test legend.position = "top"
+  p_top <- ggplot(mtcars, aes(wt, mpg, color = factor(cyl))) +
+    geom_point() +
+    theme(legend.position = "top")
+  L_top <- plotly_build(ggplotly(p_top))$x
+  expect_equal(L_top$layout$legend$orientation, "h")
+  expect_equal(L_top$layout$legend$y, 1.02)
+
+  # Test legend.position = "left"
+  p_left <- ggplot(mtcars, aes(wt, mpg, color = factor(cyl))) +
+    geom_point() +
+    theme(legend.position = "left")
+  L_left <- plotly_build(ggplotly(p_left))$x
+  expect_equal(L_left$layout$legend$xanchor, "right")
+  expect_equal(L_left$layout$legend$x, -0.15)
+
+  # Test legend.position = "none"
+  p_none <- ggplot(mtcars, aes(wt, mpg, color = factor(cyl))) +
+    geom_point() +
+    theme(legend.position = "none")
+  L_none <- plotly_build(ggplotly(p_none))$x
+  expect_false(L_none$layout$showlegend)
+
+  # Test numeric legend.position
+  p_custom <- ggplot(mtcars, aes(wt, mpg, color = factor(cyl))) +
+    geom_point() +
+    theme(legend.position = c(0.8, 0.2))
+  L_custom <- plotly_build(ggplotly(p_custom))$x
+  expect_equal(L_custom$layout$legend$x, 0.8)
+  expect_equal(L_custom$layout$legend$y, 0.2)
+})