Histograms with Fading Gradient

Visualise the distribution of a single continuous variable as a histogram with a fading alpha gradient. Counts are drawn with rounded, gradient-filled bars (like geom_col_fade() paired with ggplot2::stat_bin()). Accepts all binning parameters forwarded to ggplot2::stat_bin() (bins, binwidth, center, boundary, ...).

Usage

geom_histogram_fade(
  mapping = NULL,
  data = NULL,
  stat = "bin",
  position = "stack",
  ...,
  binwidth = NULL,
  bins = NULL,
  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

Use to override the default connection between geom_histogram_fade()/geom_freqpoly_fade() and stat_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.

binwidth

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

bins

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

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. The histogram family's vocabulary differs from geom_col_fade() / geom_bar_fade() because x is continuous (a binned variable) rather than a discrete category:

"bar" (default), "group", "fill", "colour", "global" – same meaning as in geom_col_fade().
"bin" – every bar in the same bin shares an alpha range. Useful under position = "dodge" for highlighting the tallest group within each bin (e.g. "which species dominates each Sepal.Width bin").

The "x" / "y" scopes accepted by geom_col_fade() are not available here – on a continuous binned axis they would key on round(data$x), which buckets bins by integer rounding rather than by bin identity. Use "bin" for the per-bin meaning.

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().

Aesthetics

geom_histogram_fade() understands the same aesthetics as geom_col_fade() (it is GeomHistogramFade, a subclass of GeomColFade, paired with ggplot2::stat_bin()). See ?geom_col_fade for the full aesthetics table.

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.

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

Examples

library(ggplot2)

# By default each bar has its own alpha scope
p <- ggplot(faithful, aes(waiting))
p + geom_histogram_fade()
#> `stat_bin()` using `bins = 30`. Pick better value `binwidth`.


# when all bars shall share the same alpha scope,
# set alpha_scope = "global"
p +
  geom_histogram_fade(
    alpha_scope = "global",
    alpha = 0.75,
    alpha_fade_to = 0.1,
    radius = unit(3, "pt"),
    colour = "#333333"
  ) +
  theme_minimal()
#> `stat_bin()` using `bins = 30`. Pick better value `binwidth`.


# Stacked histogram with groups
ggplot(iris, aes(Sepal.Length, fill = Species)) +
  geom_histogram_fade(alpha_fade_to = 0.25) +
  theme_minimal()
#> `stat_bin()` using `bins = 30`. Pick better value `binwidth`.


# Stacked histogram with groups and global alpha scope
ggplot(iris, aes(Sepal.Length, fill = Species)) +
  geom_histogram_fade(
    alpha_fade_to = 0.25,
    alpha_scope = "global"
  )
#> `stat_bin()` using `bins = 30`. Pick better value `binwidth`.


# Per-fill scope under position = "dodge": each fill cluster has its own
# alpha range, so the tallest sub-bar in every bin reaches full opacity.
ggplot(iris, aes(Sepal.Width, fill = Species)) +
  geom_histogram_fade(
    position = "dodge",
    bins = 10,
    alpha_scope = "fill"
  )