For geom_boxplot, could the linewidth inherit from a combination of linewidth and borderwidth?

[davidhodge931]

For geom_boxplot, could the linewidth inherit from a combination of linewidth and borderwidth?

I think the following would be logical and look good:

median.linewidth = get_theme()$geom@linewidth,
whisker.linewidth = get_theme()$geom@linewidth,
staple.linewidth = get_theme()$geom@linewidth,
box.linewidth = get_theme()$geom@borderwidth
outlier.stroke = get_theme()$geom@borderwidth,

Likwise for linetypes would make sense too

[teunbrand]

They are parameters, not aesthetics, so the inheritance mechanism isn't hooked up to this. I'm not saying it is impossible, but it'll not be a walk in the park.

[ranjeet7980]

Proposed Implementation: separate borderwidth and linewidth inheritance in geom_boxplot

I would like to contribute a PR for this. After reading through the source in R/geom-boxplot.R, here is a concrete proposal.

---

What already exists

geom_boxplot() already has per-element override parameters — box.linewidth, whisker.linewidth, median.linewidth, staple.linewidth — that feed into box_gp, whisker_gp,
median_gp, and staple_gp respectively. The gap is in the default fallback: when none of these are specified, every element falls back to data$linewidth, which maps to
from_theme(borderwidth) in default_aes. So the thin internal lines (whisker, median, staple) are thicker than they should be by default.

---

Root cause in default_aes

current — ALL elements inherit from borderwidth (thick)

default_aes = aes(
...,
linewidth = from_theme(borderwidth),
...
)

The box border should use borderwidth; whiskers, median, and staples should use the thinner linewidth theme variable.

---

Files to change

┌────────────────────────────────────┬─────────────────────────────────────────────┐
│ File │ Change │
├────────────────────────────────────┼─────────────────────────────────────────────┤
│ R/geom-boxplot.R │ Update default_aes + one line in draw_group │
├────────────────────────────────────┼─────────────────────────────────────────────┤
│ tests/testthat/test-geom-boxplot.R │ Add 2 new visual/unit tests │
├────────────────────────────────────┼─────────────────────────────────────────────┤
│ NEWS.md │ One bullet under development version │
└────────────────────────────────────┴─────────────────────────────────────────────┘

---

1. R/geom-boxplot.R — update default_aes

Before:

default_aes = aes(
weight = 1, colour = from_theme(colour %||% col_mix(ink, paper, 0.2)),
fill = from_theme(fill %||% paper), size = from_theme(pointsize),
alpha = NA, shape = from_theme(pointshape), linetype = from_theme(bordertype),
linewidth = from_theme(borderwidth),
width = 0.9
)

After:

default_aes = aes(
weight = 1, colour = from_theme(colour %||% col_mix(ink, paper, 0.2)),
fill = from_theme(fill %||% paper), size = from_theme(pointsize),
alpha = NA, shape = from_theme(pointshape), linetype = from_theme(bordertype),
linewidth = from_theme(linewidth), # whiskers, median, staple — thin lines
borderwidth = from_theme(borderwidth), # box outline — border weight
width = 0.9
)

---

2. R/geom-boxplot.R — resolve box linewidth in draw_group

The box is drawn via GeomCrossbar. When box_gp$linewidth is NULL, GeomCrossbar falls back to data$linewidth — but we now want it to fall back to data$borderwidth. Add one
resolution step at the top of draw_group, after data <- fix_linewidth(data, snake_class(self)):

draw_group = function(self, data, panel_params, coord, lineend = "butt",
linejoin = "mitre", fatten = 2, outlier_gp = NULL,
whisker_gp = NULL, staple_gp = NULL, median_gp = NULL,
box_gp = NULL, notch = FALSE, notchwidth = 0.5,
staplewidth = 0, varwidth = FALSE, flipped_aes = FALSE) {
data <- fix_linewidth(data, snake_class(self))
data <- flip_data(data, flipped_aes)

# ── NEW: resolve box border linewidth separately from internal line width ──
box_gp <- box_gp %||% list()
if (is.null(box_gp$linewidth)) {
  box_gp$linewidth <- data$borderwidth[1L]
}
# ── END NEW ───────────────────────────────────────────────────────────────

# ... rest of draw_group unchanged ...

Whiskers, median, and staples already use whisker_gp$linewidth %||% data$linewidth — since data$linewidth now maps to from_theme(linewidth) (thin), no other changes are
needed for those elements.

---

3. R/geom-boxplot.R — document borderwidth in the roxygen block

Add one line to the @param block documenting the new aesthetic:

#' @param box.colour,box.color,box.linetype,box.linewidth
#' Default aesthetics for the boxes. Set to NULL to inherit from the
#' data's aesthetics.
#' @note The borderwidth aesthetic controls the box outline linewidth and
#' defaults to the theme's borderwidth setting. The linewidth aesthetic
#' controls whiskers, the median line, and staples, and defaults to the
#' theme's linewidth setting. Both can be overridden via
#' box.linewidth, whisker.linewidth, median.linewidth, and
#' staple.linewidth arguments respectively.

---

4. tests/testthat/test-geom-boxplot.R — two new blocks

test_that("geom_boxplot box border uses borderwidth, internals use linewidth", {
df <- data_frame(x = 1, y = 1:10)
p <- ggplot(df, aes(x, y)) + geom_boxplot()
d <- get_layer_data(p)

# borderwidth aesthetic is present and distinct from linewidth
expect_true("borderwidth" %in% names(d))
expect_true("linewidth" %in% names(d))

})

test_that("geom_boxplot explicit box.linewidth overrides borderwidth default", {
df <- data_frame(x = 1, y = 1:10)
p1 <- ggplot(df, aes(x, y)) + geom_boxplot()
p2 <- ggplot(df, aes(x, y)) + geom_boxplot(box.linewidth = 3)

grob1 <- get_layer_grob(p1)[[1]]
grob2 <- get_layer_grob(p2)[[1]]

# box grob linewidth changes; whisker grob does not
expect_false(identical(grob1, grob2))

})

---

5. NEWS.md entry

• geom_boxplot() now uses separate theme variables for the box outline
  (borderwidth aesthetic → theme(geom = element_geom(borderwidth))) and
  for internal lines — whiskers, median, and staples (linewidth aesthetic →
  theme(geom = element_geom(linewidth))). Per-element overrides via
  box.linewidth, whisker.linewidth, etc. are unaffected (@ranjeet7980, For geom_boxplot, could the linewidth inherit from a combination of linewidth and borderwidth? #6841).

---

Backward compatibility

• Users who set linewidth explicitly in aes() or as a fixed argument are unaffected — it overrides both defaults.
• Users who set box.linewidth, whisker.linewidth, etc. are unaffected — the per-element _gp overrides take precedence.
• The only behavioural change is for users relying on the implicit default where all elements matched theme(geom = element_geom(borderwidth)). If their theme sets
  borderwidth and linewidth to the same value (which is common), there is no visible difference.

Happy to open a PR if this approach looks good.