Depth estimates and contours

Estimate data depth using ddalpha::depth.().

stat_depth(
  mapping = NULL,
  data = NULL,
  geom = "contour",
  position = "identity",
  contour = TRUE,
  contour_var = "depth",
  notion = "zonoid",
  notion_params = list(),
  n = 100L,
  show.legend = NA,
  inherit.aes = TRUE,
  ...
)

stat_depth_filled(
  mapping = NULL,
  data = NULL,
  geom = "contour_filled",
  position = "identity",
  contour = TRUE,
  contour_var = "depth",
  notion = "zonoid",
  notion_params = list(),
  n = 100L,
  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)).

geom

The geometric object to use to display the data for this layer. When using a stat_*() function to construct a layer, the geom argument can be used to override the default coupling between stats and geoms. The geom argument accepts the following:

A Geom ggproto subclass, for example GeomPoint.
A string naming the geom. To give the geom as a string, strip the function name of the geom_ prefix. For example, to use geom_point(), give the geom as "point".
For more information and other ways to specify the geom, see the layer geom 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.

contour

If TRUE, contour the results of the depth estimation.

contour_var

Character string identifying the variable to contour by. Can be one of "depth" or "ndepth". See the section on computed variables for details.

notion

Character; the name of the depth function (passed to ddalpha::depth.()).

notion_params

List of additional parameters passed via ... to ddalpha::depth.().

n

Number of grid points in each direction.

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.

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. borders().

...

Arguments passed on to ggplot2::geom_contour

bins

Number of contour bins. Overridden by breaks.

binwidth

The width of the contour bins. Overridden by bins.

breaks

One of:

Numeric vector to set the contour breaks
A function that takes the range of the data and binwidth as input and returns breaks as output. A function can be created from a formula (e.g. ~ fullseq(.x, .y)).

Overrides binwidth and bins. By default, this is a vector of length ten with pretty() breaks.

Value

A ggproto layer.

Details

Depth is an extension of the univariate notion of rank to bivariate (and sometimes multivariate) data (Rousseeuw &al, 1999). It comes in several flavors and is the basis for bagplots.

stat_depth() is adapted from ggplot2::stat_density_2d() and returns depth values over a grid in the same format, so it is neatly paired with ggplot2::geom_contour().

Biplot layers

ggbiplot() uses ggplot2::fortify() internally to produce a single data frame with a .matrix column distinguishing the subjects ("rows") and variables ("cols"). The stat layers stat_rows() and stat_cols() simply filter the data frame to one of these two.

The geom layers geom_rows_*() and geom_cols_*() call the corresponding stat in order to render plot elements for the corresponding factor matrix. geom_dims_*() selects a default matrix based on common practice, e.g. points for rows and arrows for columns.

Ordination aesthetics

This statistical transformation is compatible with the convenience function ord_aes().

Some transformations (e.g. stat_center()) commute with projection to the lower (1 or 2)-dimensional biplot space. If they detect aesthetics of the form ..coord[0-9]+, then ..coord1 and ..coord2 are converted to x and y while any remaining are ignored.

Other transformations (e.g. stat_spantree()) yield different results in a lower-dimensional biplot when they are computed before versus after projection. If the stat layer detects these aesthetics, then the transformation is performed before projection, and the results in the first two dimensions are returned as x and y.

A small number of transformations (stat_rule()) are incompatible with ordination aesthetics but will accept ord_aes() without warning.

Computed variables

These are calculated during the statistical transformation and can be accessed with delayed evaluation.

stat_depth() and stat_depth_filled() compute different variables depending on whether contouring is turned on or off. With contouring off (contour = FALSE), both stats behave the same, and the following variables are provided:

depth: the depth estimate
ndepth: depth estimate, scaled to a maximum of 1

With contouring on (contour = TRUE), either ggplot2::stat_contour() or ggplot2::stat_contour_filled() is run after the depth estimate has been obtained, and the computed variables are determined by these stats.

References

Rousseeuw PJ, Ruts I, & Tukey JW (1999) "The Bagplot: A Bivariate Boxplot". The American Statistician, 53(4): 382–387. doi:10.1080/00031305.1999.10474494

Examples

# base Motor Trends plot
b <- ggplot(mtcars, aes(wt, disp)) + geom_point()

# depth raster
b + geom_raster(stat = "depth", aes(fill = after_stat(depth)))

# depth grid
b + stat_depth(
  geom = "point", contour = FALSE,
  aes(size = after_stat(depth)), n = 20
)


# depth contours
b + geom_contour(stat = "depth", contour = TRUE)

# depth bands
b + geom_contour_filled(stat = "depth_filled", contour = TRUE, alpha = .75)

# contours colored by group
b + stat_depth(aes(color = factor(cyl)))

# custom depth notion
b + stat_depth(
  aes(color = factor(cyl)),
  notion = "halfspace", notion_params = list(exact = TRUE)
)


# contours faceted by group
b + stat_depth_filled(alpha = .75) +
  facet_wrap(facets = vars(factor(cyl)))

# scaled to the unit interval
# FIXME: Some polygons are missing.
b + stat_depth_filled(contour_var = "ndepth", alpha = .75) +
  facet_wrap(facets = vars(factor(cyl)))