Ridgeline Plots with Fading Gradient

geom_ridgeline_fade() draws ridgeline plots: multiple area shapes stacked at different vertical offsets and adds a vertical alpha gradient that fades from opaque at the peaks to transparent at each ridge's baseline.

The gradient machinery is shared with geom_area_fade(); the difference is that each group's baseline is its own y value rather than zero, enabling the characteristic overlapping-ridges layout.

geom_ridgeline_density_fade() is a convenience wrapper around geom_ridgeline_fade() that uses ggplot2::stat_density() to compute a kernel density estimate, mapping the result to height automatically via aes(height = after_stat(density)). Adjust scale manually so adjacent ridges reach the desired overlap.

geom_ridgeline_freqpoly_fade() and geom_ridgeline_histogram_fade() are the ridgeline counterparts of geom_freqpoly_fade() and geom_histogram_fade(). Both bin the x aesthetic per categorical y baseline; they differ only in how consecutive bins connect: freqpoly draws linear connections between bin midpoints (polyline); histogram draws stepped flat-top rectangles with vertical jumps at the bin edges.

Usage

geom_ridgeline_fade(
  mapping = NULL,
  data = NULL,
  stat = "identity",
  position = NULL,
  ...,
  alpha_fade_to = 0,
  alpha_scope = "group",
  scale = NULL,
  min_height = NULL,
  na.rm = FALSE,
  orientation = NA,
  show.legend = NA,
  inherit.aes = TRUE
)

geom_ridgeline_density_fade(
  mapping = NULL,
  data = NULL,
  stat = "density",
  ...,
  alpha_fade_to = 0,
  alpha_scope = "group",
  scale = NULL,
  min_height = NULL,
  na.rm = FALSE,
  orientation = NA,
  show.legend = NA,
  inherit.aes = TRUE
)

geom_ridgeline_freqpoly_fade(
  mapping = NULL,
  data = NULL,
  position = NULL,
  ...,
  bins = 30,
  binwidth = NULL,
  center = NULL,
  boundary = NULL,
  closed = c("right", "left"),
  pad = TRUE,
  alpha_fade_to = 0,
  alpha_scope = "group",
  scale = NULL,
  min_height = NULL,
  na.rm = FALSE,
  orientation = NA,
  show.legend = NA,
  inherit.aes = TRUE
)

geom_ridgeline_histogram_fade(
  mapping = NULL,
  data = NULL,
  position = NULL,
  ...,
  bins = 30,
  binwidth = NULL,
  center = NULL,
  boundary = NULL,
  closed = c("right", "left"),
  pad = TRUE,
  alpha_fade_to = 0,
  alpha_scope = "group",
  scale = NULL,
  min_height = NULL,
  na.rm = FALSE,
  orientation = NA,
  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. 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.

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.

...

Additional arguments passed to geom_ridgeline_fade(), including smoothing parameters (bw, adjust, kernel, n, trim, bounds) forwarded to ggplot2::stat_density().

alpha_fade_to

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

alpha_scope

How to scale alpha across ridges. Vocabulary aligned with geom_area_fade():

• "group" (default): every ridge independently uses the full alpha range from alpha_fade_to to full opacity. Each ridge is its own reference.

• "global": alpha is scaled relative to the tallest ridge in the entire layer, including across facet panels. Shorter ridges fade in proportion.

scale

Height multiplier applied to height. The default NULL auto-scales the layer so the tallest ridge overlaps its neighbour by ~50% (to 2 / max(abs(height))). The auto-resolved value is reported via cli::cli_inform() so you have a starting point if you want to override.

min_height

Minimum height value to draw. Points with height < min_height are dropped, creating gaps in the ridgeline. Defaults to 0.

na.rm

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

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.

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().

bins

Number of bins. Overridden by binwidth. Defaults to 30. Forwarded to ggplot2::stat_bin().

binwidth

Width of each bin in data units. When supplied, takes precedence over bins. Forwarded to ggplot2::stat_bin().

center, boundary, closed, pad

Forwarded to ggplot2::stat_bin(). pad = TRUE (the default for ridgeline forms) prepends/appends a zero-count bin at each end so the ribbon touches the baseline cleanly.

Value

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

Coordinate systems

geom_ridgeline_fade() only supports linear gradients. When used with ggplot2::coord_polar() or ggplot2::coord_radial(), the geom falls back to standard ridgeline rendering (equivalent to ggplot2::geom_ribbon()), which means no gradient fill is added. The geom emits a warning in this case.

Legend key order

Ridges are rendered back-to-front: the ridge with the highest y-baseline is drawn first (furthest back) and the ridge with the lowest y-baseline is drawn last (on top). When fill tracks y, the default fill legend lists levels in ascending order – placing the lowest y at the top of the legend – which is the reverse of the spatial top-to-bottom reading order (highest y at top of chart, lowest y at bottom).

To align the legend with the chart, reverse the legend keys:

+ guides(fill = guide_legend(reverse = TRUE))

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. (2021). "Luminance Masks in R Graphics." Technical Report 2021-04, Department of Statistics, The University of Auckland. Version 1. https://www.stat.auckland.ac.nz/~paul/Reports/GraphicsEngine/masks/masks.html

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

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_ridgeline_density_fade() for the convenience density-ridgeline wrapper, geom_area_fade() for area plots with the same gradient effect.

geom_ridgeline_fade() for the lower-level geom. This wrapper mirrors the design of ggridges::geom_density_ridges() from the ggridges package by Claus O. Wilke, which is the full-featured original; ggpointless adds the alpha-gradient rendering layer.

Aesthetics

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

•	x
•	y
•	height
•	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)

totals <- aggregate(
  sales ~ year + month,
  data = subset(txhousing, year <= 2004),
  FUN = sum,
  na.rm = TRUE
)

p <- ggplot(totals, aes(x = month, y = year, group = year, height = sales))
p + geom_ridgeline_fade(outline.type = "none")
#> ℹ Using auto-computed `scale = 0.000068`.
#> • Pass an explicit `scale` to override.


# increase overlap using the scale parameter
p + geom_ridgeline_fade(outline.type = "none", scale = 0.0001)


# flip orientation
p + aes(y = month, x = year) +
  geom_ridgeline_fade()
#> ℹ Using auto-computed `scale = 0.000068`.
#> • Pass an explicit `scale` to override.


# Map a variable to `fill` to get a 2D gradient
# and use stat_chaikin to smooth curves
p +
  geom_ridgeline_fade(
    aes(fill = after_stat(height)),
    alpha_scope = "global",
    outline.type = "none",
    stat = "chaikin"
  )
#> ℹ Using auto-computed `scale = 0.000069`.
#> • Pass an explicit `scale` to override.


# Average monthly temperatures at Nottingham, 1920-1939
dmn <- list(
  month.abb,
  time(datasets::nottem) |> floor() |> unique()
)

df_nottem <- datasets::nottem |>
  matrix(data = _, 12, dimnames = dmn) |>
  as.data.frame() |>
  stack() |>
  cbind(month = factor(month.abb, levels = month.abb))

# Shared base plot reused by the three ridgeline variants below
p <- ggplot(df_nottem, aes(x = values, y = month)) +
  labs(
    x = NULL,
    y = NULL,
    caption = "Average air temperatures at Nottingham Castle in degrees Fahrenheit (1920-1939)"
  ) +
  scale_fill_gradient(low = "navy", high = "tomato", guide = "none") +
  scale_y_discrete(expand = expansion(add = c(0.2, 1.2)))

# Density ridgelines -- convenience wrapper for the stat = "density"
p +
  geom_ridgeline_density_fade(
    aes(fill = after_stat(x)),
    outline.type = "none"
  )
#> ℹ Using auto-computed `scale = 6.5`.
#> • Pass an explicit `scale` to override.


# freqpoly uses stat = "bin"
p +
  geom_ridgeline_freqpoly_fade(
    aes(fill = after_stat(x)),
    alpha_scope = "global",
    bins = 40
  )
#> ℹ Using auto-computed `scale = 0.29`.
#> • Pass an explicit `scale` to override.

# ridgeline histogram uses stat = "bin" too
p +
  geom_ridgeline_histogram_fade(
    aes(fill = after_stat(x)),
    alpha_scope = "global",
    bins = 40
  )
#> ℹ Using auto-computed `scale = 0.29`.
#> • Pass an explicit `scale` to override.