Rectangles with a Fading Gradient and Rounded Corners

geom_rect_fade() draws axis-aligned rectangles and fills each one with a linear gradient that fades one edge to transparent. The direction is controlled by fade_direction. Corners can be rounded via the radius argument, enabling rounded rectangles and smooth-cornered visual elements. The default of 0 pt produces plain rectangles.

Usage

geom_rect_fade(
  mapping = NULL,
  data = NULL,
  stat = "identity",
  position = "identity",
  ...,
  alpha_fade_to = 0,
  fade_direction = "vertical",
  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

Use to override the default connection between geom_rect_fade() and stat_identity().

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 fading edge of each rectangle. Defaults to 0 (fully transparent).

fade_direction

Direction of the alpha gradient. One of:

"vertical": (default) Top edge is opaque (ymax), bottom edge fades to alpha_fade_to (ymin).
"horizontal": Left edge is opaque (xmin), right edge fades to alpha_fade_to (xmax).

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") (sharp corners).

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().

Polar coordinates

Under ggplot2::coord_polar() / ggplot2::coord_radial() each rectangle is bent into an annular segment. A radial alpha gradient – transparent at the inner radius, opaque at the outer – is rendered when the fade direction aligns with the radial axis:

theta = "x" (default) + fade_direction = "vertical": ymin/ymax map to inner/outer radius and fade radially.
theta = "y" + fade_direction = "horizontal": xmin/xmax map to inner/outer radius and fade radially.

Any other combination (for example theta = "x" with fade_direction = "horizontal") would require an angular / conic gradient, which grid does not yet expose. Such plots fall back to plain ggplot2::geom_rect() rendering and emit a one-time warning. Rounded corners (radius) are ignored in polar coordinates since arcs do not carry corner geometry.

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

•	`x` or `width` or `xmin` or `xmax`
•	`y` or `height` or `ymin` or `ymax`
•	`alpha`	→ `NA`
•	`colour`	→ via `theme()`
•	`fill`	→ via `theme()`
•	`group`	→ inferred
•	`linetype`	→ via `theme()`
•	`linewidth`	→ via `theme()`

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

Examples

library(ggplot2)

# With geom_rect_fade() you can draw arbitrary rectangles
ggplot(head(economics, 25), aes(date, unemploy)) +
  geom_rect_fade(
    data = data.frame(
      xmin = as.Date("1968-07-01"),
      xmax = as.Date("1969-07-01"),
      ymin = -Inf, ymax = 2800
    ),
    inherit.aes = FALSE,
    alpha = 0,
    alpha_fade_to = 0.3,
    aes(xmin = xmin, xmax = xmax, ymin = ymin, ymax = ymax)
  ) +
  stat_fourier(geom = "line_fade", fade_direction = "start", alpha_fade_to = 0.2) +
  geom_point(size = 3, alpha = 0.2) +
  theme_minimal()