Bar Charts with Fading Gradient and Rounded Corners

geom_col_fade() and geom_bar_fade() draw bar charts like ggplot2::geom_col() / ggplot2::geom_bar() but with options to add an alpha gradient that fades from opaque at the peak of each bar to transparent at its baseline; and rounded corners are supported via grid::roundrectGrob(), controlled by the radius argument.

Usage

geom_col_fade(
  mapping = NULL,
  data = NULL,
  stat = "identity",
  position = "stack",
  ...,
  alpha_fade_to = 0,
  alpha_scope = "bar",
  orientation = NA,
  radius = grid::unit(0, "pt"),
  lineend = "butt",
  linejoin = "mitre",
  na.rm = FALSE,
  show.legend = NA,
  inherit.aes = TRUE
)

geom_bar_fade(
  mapping = NULL,
  data = NULL,
  stat = "count",
  position = "stack",
  ...,
  alpha_fade_to = 0,
  alpha_scope = "bar",
  orientation = NA,
  radius = grid::unit(0, "pt"),
  lineend = "butt",
  linejoin = "mitre",
  na.rm = FALSE,
  show.legend = NA,
  inherit.aes = TRUE
)

Arguments

mapping

Set of aesthetic mappings created by aes(). If specified and inherit.aes = TRUE (the default), it is combined with the default mapping at the top level of the plot. You must supply mapping if there is no plot mapping.

data

The data to be displayed in this layer. There are three options:

If NULL, the default, the data is inherited from the plot data as specified in the call to ggplot().

A data.frame, or other object, will override the plot data. All objects will be fortified to produce a data frame. See fortify() for which variables will be created.

A function will be called with a single argument, the plot data. The return value must be a data.frame, and will be used as the layer data. A function can be created from a formula (e.g. ~ head(.x, 10)).

stat

The statistical transformation to use on the data for this layer, as a string. geom_col_fade() uses "identity" (no transform), geom_bar_fade() uses "count", and geom_histogram_fade() uses "bin".

position

A position adjustment to use on the data for this layer. This can be used in various ways, including to prevent overplotting and improving the display. The position argument accepts the following:

The result of calling a position function, such as position_jitter(). This method allows for passing extra arguments to the position.
A string naming the position adjustment. To give the position as a string, strip the function name of the position_ prefix. For example, to use position_jitter(), give the position as "jitter".
For more information and other ways to specify the position, see the layer position documentation.

...

Other arguments passed on to layer()'s params argument. These arguments broadly fall into one of 4 categories below. Notably, further arguments to the position argument, or aesthetics that are required can not be passed through .... Unknown arguments that are not part of the 4 categories below are ignored.

Static aesthetics that are not mapped to a scale, but are at a fixed value and apply to the layer as a whole. For example, colour = "red" or linewidth = 3. The geom's documentation has an Aesthetics section that lists the available options. The 'required' aesthetics cannot be passed on to the params. Please note that while passing unmapped aesthetics as vectors is technically possible, the order and required length is not guaranteed to be parallel to the input data.
When constructing a layer using a stat_*() function, the ... argument can be used to pass on parameters to the geom part of the layer. An example of this is stat_density(geom = "area", outline.type = "both"). The geom's documentation lists which parameters it can accept.
Inversely, when constructing a layer using a geom_*() function, the ... argument can be used to pass on parameters to the stat part of the layer. An example of this is geom_area(stat = "density", adjust = 0.5). The stat's documentation lists which parameters it can accept.
The key_glyph argument of layer() may also be passed on through .... This can be one of the functions described as key glyphs, to change the display of the layer in the legend.

alpha_fade_to

A single finite number between 0 and 1. The alpha value at the baseline of each bar. Defaults to 0 (fully transparent).

alpha_scope

How to choose the per-bar reference height that the gradient normalises against. One of:

"bar" (default): each bar uses its own height – every peak hits full opacity.
"global": every bar normalises to the tallest bar in the entire layer, including across facet panels. The single tallest bar reaches full opacity; all others fade in proportion.
"x" / "y": every bar normalises to the tallest bar at the same discrete x (or y) position. Useful for highlighting the leader within a stack or dodge cluster. "x" is for vertical bars; "y" is for horizontal bars (coord_flip() / orientation = "y"). Mismatched orientation aborts with a hint.
"group": every bar normalises to the tallest bar in its ggplot2 group (data$group – the interaction of all discrete aesthetics). Most useful with explicit aes(group = ...); with the typical aes(x = factor, fill = factor) layout it degenerates to "bar" because every (x, fill) pair is its own group.
"fill" / "colour": every bar normalises to the tallest bar with the same fill (or colour). Bars sharing a fill share an alpha range across x and across facet panels.

orientation

The orientation of the layer. The default (NA) automatically determines the orientation from the aesthetic mapping. In the rare event that this fails it can be given explicitly by setting orientation to either "x" or "y". See the Orientation section for more detail.

radius

Corner radius passed to grid::roundrectGrob(). A grid::unit() object (e.g. unit(4, "pt")); a bare number is interpreted as points. Defaults to unit(0, "pt") (matching geom_bar() / geom_col()).

lineend

Line end style (round, butt, square).

linejoin

Line join style (round, mitre, bevel).

na.rm

If FALSE, the default, missing values are removed with a warning. If TRUE, missing values are silently removed.

show.legend

logical. Should this layer be included in the legends? NA, the default, includes if any aesthetics are mapped. FALSE never includes, and TRUE always includes. It can also be a named logical vector to finely select the aesthetics to display. To include legend keys for all levels, even when no data exists, use TRUE. If NA, all levels are shown in legend, but unobserved levels are omitted.

inherit.aes

If FALSE, overrides the default aesthetics, rather than combining with them. This is most useful for helper functions that define both data and aesthetics and shouldn't inherit behaviour from the default plot specification, e.g. annotation_borders().

Value

A ggplot2::layer() object that can be added to a ggplot2::ggplot().

Orientation

This geom treats each axis differently and, thus, can thus have two orientations. Often the orientation is easy to deduce from a combination of the given mappings and the types of positional scales in use. Thus, ggplot2 will by default try to guess which orientation the layer should have. Under rare circumstances, the orientation is ambiguous and guessing may fail. In that case the orientation can be specified directly using the orientation parameter, which can be either "x" or "y". The value gives the axis that the geom should run along, "x" being the default orientation you would expect for the geom.

alpha_scope = "global" under faceting

alpha_scope = "global" ties opacity to absolute height across the whole layer, so two ridges / areas / bars of equal height render at equal alpha regardless of which panel they're in. This is meaningful only when panels share a common y scale. Under facet_wrap(scales = "free_y") (or facet_grid(rows = ..., scales = "free")) each panel rescales y independently, so the visual height of a shape no longer reflects its data height; the alpha encoding then conflicts with what the eye reads from the panel size. For comparable alpha across free-y panels you have two options: stick to the default scales = "fixed", or accept that under free scales alpha_scope = "group" is the more honest choice (each shape independently uses its own alpha range).

Legend key under coord_flip

The legend key glyph always shows the canonical (data-axis) fade direction – vertical for the default orientation, horizontal under orientation = "y". Under ggplot2::coord_flip() the rendered geom rotates correctly but the legend key does not: ggplot2's legend builder is coord-independent by design (draw_key has no access to the coord). For a legend key that matches a horizontal layout, prefer aes(y = ...) with auto-detected orientation = "y" over aes(x = ...) + coord_flip().

References

Murrell, P. (2022). "Vectorised Pattern Fills in R Graphics." Technical Report 2022-01, Department of Statistics, The University of Auckland. Version 1. https://www.stat.auckland.ac.nz/~paul/Reports/GraphicsEngine/vecpat/vecpat.html

Aesthetics

geom_col_fade() understands the following aesthetics. Required aesthetics are displayed in bold and defaults are displayed for optional aesthetics:

•	`x`
•	`y`
•	`alpha`	→ `NA`
•	`colour`	→ via `theme()`
•	`fill`	→ via `theme()`
•	`group`	→ inferred
•	`linetype`	→ via `theme()`
•	`linewidth`	→ via `theme()`
•	`width`	→ `0.9`

Learn more about setting these aesthetics in vignette("ggplot2-specs").

Examples

library(ggplot2)

df <- data.frame(
  x = c("A", "B", "C", "D", "E"),
  y = c(3, 4, -2, -0.5, 1)
)

ggplot(df, aes(x, y)) +
  geom_col_fade() +
  theme_minimal()


# Rounded bar charts are supported too
ggplot(df, aes(x, y)) +
  geom_col_fade(radius = unit(10, "pt")) +
  theme_minimal()


# Start at 90% opacity and keep some opacity at the baseline
ggplot(df, aes(x, y)) +
  geom_col_fade(
    alpha = 0.9,
    alpha_fade_to = 0.1
  ) +
  theme_minimal()


# Horizontal bars are supported
ggplot(df, aes(y, x)) +
  geom_col_fade() +
  theme_minimal()



# Multiple groups with different alpha scopes
p <- ggplot(diamonds, aes(color, fill = cut)) +
  scale_fill_viridis_d(guide = "none") +
  labs(x = NULL, y = NULL) +
  theme_minimal()

# By default each bar has its own alpha scope
p + geom_bar_fade()


# With alpha_scope = "x", bars at same x aesthetic share same alpha scope
p + geom_bar_fade(alpha_scope = "x")


# With alpha_scope = "fill", alpha is defined by what is mapped to the fill aesthetic
p + geom_bar_fade(alpha_scope = "fill")


# With alpha_scope = "global", the maximum absolute
# value across the whole dataset is fully opaque
p + geom_bar_fade(alpha_scope = "global")


# same examples for position dodge with varying alpha scope:
p + geom_bar_fade(alpha_scope = "bar", position = "dodge")

p + geom_bar_fade(alpha_scope = "x", position = "dodge")

p + geom_bar_fade(alpha_scope = "fill", position = "dodge")

p + geom_bar_fade(alpha_scope = "global", position = "dodge")


# Polar coordinates are supported too, if you need it
ggplot(diamonds, aes(x = factor(1), fill = cut)) +
  geom_bar_fade(width = 1) +
  coord_polar(theta = "y") +
  theme_void()