Reference Lines with a Fading Gradient

These geoms draw reference lines – horizontal, vertical, or diagonal – with an alpha gradient along the line so that one or both ends fade to transparent.

Like their ggplot2 counterparts, these geoms can be used as annotations by passing slope/intercept, yintercept, or xintercept as arguments directly. In that case the line is constant across facets. To vary lines across facets, supply your own data and mapping.

Usage

geom_abline_fade(
  mapping = NULL,
  data = NULL,
  stat = "identity",
  ...,
  slope,
  intercept,
  alpha_fade_to = 0,
  fade_direction = "start",
  na.rm = FALSE,
  show.legend = NA,
  inherit.aes = FALSE
)

geom_hline_fade(
  mapping = NULL,
  data = NULL,
  stat = "identity",
  position = "identity",
  ...,
  yintercept,
  alpha_fade_to = 0,
  fade_direction = "start",
  na.rm = FALSE,
  show.legend = NA,
  inherit.aes = FALSE
)

geom_vline_fade(
  mapping = NULL,
  data = NULL,
  stat = "identity",
  position = "identity",
  ...,
  xintercept,
  alpha_fade_to = 0,
  fade_direction = "start",
  na.rm = FALSE,
  show.legend = NA,
  inherit.aes = FALSE
)

Arguments

mapping

Set of aesthetic mappings created by ggplot2::aes().

data

A data frame, or other object, to override the plot data.

stat

The statistical transformation to use on the data for this layer. When using a geom_*() function to construct a layer, the stat argument can be used to override the default coupling between geoms and stats. The stat argument accepts the following:

• A Stat ggproto subclass, for example StatCount.

• A string naming the stat. To give the stat as a string, strip the function name of the stat_ prefix. For example, to use stat_count(), give the stat as "count".

• For more information and other ways to specify the stat, see the layer stat 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 end(s). Defaults to 0 (fully transparent).

fade_direction

Which end(s) of the line fade. A character vector containing "start", "end", or both c("start", "end"). Defaults to "start". For horizontal lines, "start" fades the visual left end; for vertical lines, "start" fades the visual bottom end; for diagonal lines, "start" fades toward the visual left edge of the panel. See Gradient direction and reversed scales for behaviour under ggplot2::scale_x_reverse() / ggplot2::scale_y_reverse().

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().

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.

xintercept, yintercept, slope, intercept

Parameters that control the position of the line. If these are set, data, mapping and show.legend are overridden.

Value

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

Details

Gradient direction and reversed scales

The fade direction is defined in visual (panel) space, not in data space. fade_direction = "start" always makes the start of the line transparent – for horizontal lines this is the visual left edge; for vertical lines, the visual bottom edge – regardless of whether the x or y scale is reversed. So fade_direction = "start" means "transparent at the left/bottom panel edge" irrespective of axis direction.

If you do want the gradient to track the data direction – making the low-value end transparent even when it appears on the visual right – pass fade_direction = "end" to override the default:

# On a reversed x-axis, "end" fades toward larger x values (visual left)
p + scale_x_reverse() +
  geom_hline_fade(yintercept = 20, fade_direction = "end")

Non-linear coordinate systems (coord_polar / coord_radial)

Under non-linear coordinate systems the "line" is conceptually a curve in device space:

• geom_hline_fade() traces a circle at the given yintercept (constant radius).

• geom_vline_fade() traces a ray at the given xintercept (constant angle).

• geom_abline_fade() traces a curve that spirals or arcs based on the slope/intercept.

The fade is applied along the path length of that curve, so fade_direction = "start" fades the beginning of the traced path. For an hline_fade() circle the "start" is the first vertex of the traced circle (at the leftmost angle of the coord system, 12 o'clock by default), which may not match intuition from cartesian use – pass fade_direction = "end" to reverse.

Internally, the line is subdivided into a dense set of vertices in data space before the coord transform so the curve appears smooth. This is transparent to the user but explains why the grob carries more vertices than in the linear case.

References

Murrell, P., Pedersen, T. L., and Skintzos, P. (2023). "Porter-Duff Compositing Operators in R Graphics." Department of Statistics, The University of Auckland. Version 1. https://www.stat.auckland.ac.nz/~paul/Reports/GraphicsEngine/compositing/compositing.html

Murrell, P. (2023). "Groups, Compositing Operators, and Affine Transformations in R Graphics." Technical Report 2021-02, Department of Statistics, The University of Auckland. Version 3. https://www.stat.auckland.ac.nz/~paul/Reports/GraphicsEngine/groups/groups.html

See also

geom_segment_fade() for fading line segments with explicit endpoints, geom_path_fade() and geom_line_fade() for connected paths; ggplot2::geom_abline(), ggplot2::geom_hline(), and ggplot2::geom_vline() for their ggplot2 originals.

Aesthetics

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

•	x
•	y
•	xend
•	yend
•	alpha	→ NA
•	colour	→ via theme()
•	group	→ inferred
•	linetype	→ via theme()
•	linewidth	→ via theme()

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

Examples

library(ggplot2)

p <- ggplot(mtcars, aes(wt, mpg)) + geom_point()

# Horizontal reference line, fading from the left
p + geom_hline_fade(yintercept = 20, linewidth = 1.5)


# Vertical line fading at both ends
p + geom_vline_fade(xintercept = 3, linewidth = 1.5,
                    fade_direction = c("start", "end"))


# Diagonal line of best fit, fading from the left
coefs <- coef(lm(mpg ~ wt, data = mtcars))
p + geom_abline_fade(intercept = coefs[1], slope = coefs[2], linewidth = 1.5)