Facet: Facets
In ggplot2: Create Elegant Data Visualisations Using the Grammar of Graphics

Facet

R Documentation

Description

All ⁠facet_*()⁠ functions returns a Facet object or an object of a Facet subclass. This object describes how to assign data to different panels, how to apply positional scales and how to lay out the panels, once rendered.

Details

Extending facets can range from the simple modifications of current facets, to very laborious rewrites with a lot of gtable() manipulation.For some examples of both, please see the extension vignette. The object and its parameters are chaperoned by the Layout class.

Facet subclasses, like other extendible ggproto classes, have a range of methods that can be modified. Some of these are required for all new subclasses, while other only need to be modified if need arises.

The required methods are:

compute_layout
map_data()
draw_panels() or its subsidiaries:
- init_gtable()
- attach_axes()
- attach_strips()

In addition to the methods above, it can be useful to override the default behaviour of one or more of the following methods:

setup_params()
init_scales()
train_scale()
finish_data()
draw_back(), draw_front() or draw_labels()

All extension methods receive the content of the params field as the params argument, so the constructor function will generally put all relevant information into this field.

Fields

shink

A scalar boolean which when TRUE, will shrink scales to fit output statistics rather than raw data. If FALSE, will only include raw data before statistical summary. By exception this is not part of the params field.

params

A named list of parameters populated by the constructor function.

setup_params

Description

A function method for modifying or checking the parameters based on the data. The default method includes a .possible_columns variable giving column names.

Usage

Facet$setup_params(data, params)

Arguments

data: A list of data frames. The first item is the global data, which is followed by layer data in subsequent items.
params: A list of current parameters.

Value

A list of parameters

setup_data

Description

A function method for modifying or checking the data prior to adding defaults. The default method returns data unaltered.

Usage

Facet$setup_data(data, params)

Arguments

data: A list of data frames. The first item is the global data, which is followed by layer data in subsequent items.
params: A list of parameters coming from the setup_params() method.

Value

A list of data frames of the same length as the data argument

compute_layout

Description

A function method for creating the correspondence between faceting variable levels, panels and position scales. It places panels like cells in a matrix.

Usage

Facet$compute_layout(data, params)

Arguments

data: A list of data frames. The first item is the global data, which is followed by layer data in subsequent items.
params: A list of parameters coming from the setup_params() method.

Value

A data frame with 1 row per panel, containing at least integer columns ROW, COL, PANEL, SCALE_X and SCALE_Y. Can contain additional information in terms of columns, typically faceting variables.

map_data

Description

A function method for to create the PANEL variable in layer data. The PANEL variable is a special variable that tracks the relationship between rows in the layer data and the panels described in the layout input.

In addition, #' this function may copy or discard rows as needed, for example when adding margins in FacetGrid.

Usage

Facet$map_data(data, layout, params)

Arguments

data: A list of data frames containing layer data.
layout: A data frame computed by the compute_layout() method. Typically contains the faceting variables, ROW, COL, PANEL, SCALE_X and SCALE_Y variables.
params: A list of parameters coming from the setup_params() method.

Value

A list of data frames containing layer data including a PANEL variable.

init_scales

Description

A function method for initialising position scales. Given a prototype scale for x and y, creates layout specific scales to accommodate the relationships between panels and scales. By default, the prototype scales are cloned for each SCALE_X and SCALE_Y level. The function is called separately; once for x and once for y.

Usage

Facet$init_scales(layout, x_scale, y_scale, params)

Arguments

layout: A data frame computed by the compute_layout() method. Typically contains the faceting variables, ROW, COL, PANEL, SCALE_X and SCALE_Y variables.
x_scale,y_scale: A position scale for the x and y aesthetics respectively.
params: A list of parameters coming from the setup_params() method.

Value

A named list with x and y elements containing a list of panel scales for each SCALE_X and/or SCALE_Y level respectively.

train_scales

Description

A function method for training position scales. The default trains each scale on the data related to its panels.

Usage

Facet$train_scales(x_scales, y_scales, layout, data, params)

Arguments

x_scales,y_scales: A list of panel scales for each SCALE_X and SCALE_Y level respectively.
layout: A data frame computed by the compute_layout() method. Typically contains the faceting variables, ROW, COL, PANEL, SCALE_X and SCALE_Y variables.
data: A list of data frames containing layer data.
params: A list of parameters coming from the setup_params() method.

Value

Nothing, this method is called for its side-effect of training the scales.

setup_panel_params

Description

A function method as a hook to give facets input over panel parameters. By default, returns panel parameters unaltered.

Usage

Facet$setup_panel_params(panel_params, coord, ...)

Arguments

panel_params: A named list of view scales, ranges and other optional parameters from Coord$setup_panel_params().
coord: A ⁠<Coord>⁠ ggproto object.
...: Currently not in use. For future expansion.

Value

A list of panel parameters.

finish_data

Description

A function method as a hook for making last-minute modifications to layer data before it is rendered by Geoms. The default is to not modify the data.

Usage

Facet$finish_data(data, layout, x_scales, y_scales, params)

Arguments

data: A data frame containing layer data of a single layer.
layout: A data frame computed by the compute_layout() method. Typically contains the faceting variables, ROW, COL, PANEL, SCALE_X and SCALE_Y variables.
x_scales,y_scales: A list of panel scales for each SCALE_X and SCALE_Y level respectively.
params: A list of parameters coming from the setup_params() method.

Value

A data frame containing layer data.

draw_panel_content

Description

A function method to assemble the panel contents. It delegates the draw_back() and draw_front() methods, as well as Coord$draw_panel().

Usage

Facet$draw_panel_content(
  panels,
  layout,
  x_scales,
  y_scales,
  ranges,
  coord,
  theme,
  params,
  ...
)

Arguments

panels: A list parallel to layers. Each element is another list with grobs for each panel, generated by Layer$draw_geom().
layout: A data frame computed by the compute_layout() method. Typically contains the faceting variables, ROW, COL, PANEL, SCALE_X and SCALE_Y variables.
x_scales,y_scales: A list of panel scales for each SCALE_X and SCALE_Y level respectively.
ranges: A list of panel parameters from the setup_panel_params() augmented with position guides.
coord: A ⁠<Coord>⁠ ggproto object.
data: A list of data frames containing layer data.
theme: A complete theme object.
params: A list of parameters coming from the setup_params() method.
...: Currently not in use.

Value

A list of grobs, one for each level of the PANEL layout variable. Grob can be zeroGrob() to draw nothing.

draw_back,draw_front

Description

A function method draw facet background (back) and foreground (front) for panels. The front and back will sandwich the grobs created by layers. The default methods draw nothing.

Usage

Facet$draw_back(data, layout, x_scales, y_scales, theme, params)
Facet$draw_front(data, layout, x_scales, y_scales, theme, params)

Arguments

data: A list of data frames containing layer data.
layout: A data frame computed by the compute_layout() method. Typically contains the faceting variables, ROW, COL, PANEL, SCALE_X and SCALE_Y variables.
x_scales,y_scales: A list of panel scales for each SCALE_X and SCALE_Y level respectively.
theme: A complete theme object.
params: A list of parameters coming from the setup_params() method.

Value

A list of grobs, one for each level of the PANEL layout variable. Grob can be zeroGrob() to draw nothing.

draw_panels

Description

A function method that orchestrates the majority of facet drawing. It is responsible for assembling a gtable with panel content decorated with axes and strips. The resulting gtable is the basis for the plot in its entirety. It delegates these tasks to the init_gtable(), attach_axes() and attach_strips() methods.

Usage

Facet$draw_panels(
  panels,
  layout,
  x_scales,
  y_scales,
  ranges,
  coord,
  data,
  theme,
  params
)

Arguments

panels: A list of grobs, one per panel.
layout: A data frame computed by the compute_layout() method. Typically contains the faceting variables, ROW, COL, PANEL, SCALE_X and SCALE_Y variables.
x_scales,y_scales: A list of panel scales for each SCALE_X and SCALE_Y level respectively.
ranges: A list of panel parameters from the setup_panel_params() augmented with position guides.
coord: A ⁠<Coord>⁠ ggproto object.
data: A list of data frames containing layer data.
theme: A complete theme object.
params: A list of parameters coming from the setup_params() method.

Value

A gtable object.

init_gtable

Description

A function method that initiates a gtable object containing panels set at the appropriate ROW and COL cells from the layout. The panels are separated by the ⁠panel.spacing.{x/y}⁠ spacing.

Usage

Facet$init_gtable(panels, layout, theme, ranges, params, aspect_ratio)

Arguments

panels: A list of grobs, one per panel.
layout: A data frame computed by the compute_layout() method. Typically contains the faceting variables, ROW, COL, PANEL, SCALE_X and SCALE_Y variables.
theme: A complete theme object.
ranges: A list of panel parameters from the setup_panel_params() augmented with position guides.
aspect_ratio: A scalar numeric for the panel aspect ratio or NULL for no aspect ratio.

Value

A gtable object containing panel grobs prefixed with "panel".

attach_axes

Description

A function method that renders position guides (axes) and attaches these to the gtable with panels. The default method returns the gtable unaltered.

Usage

Facet$attach_axes(table, layout, ranges, coord, theme, params)

Arguments

table: A gtable object populated with panels from the init_gtable() method.
layout: A data frame computed by the compute_layout() method. Typically contains the faceting variables, ROW, COL, PANEL, SCALE_X and SCALE_Y variables.
ranges: A list of panel parameters from the setup_panel_params() augmented with position guides.
coord: A ⁠<Coord>⁠ ggproto object.
theme: A complete theme object.
params: A list of parameters coming from the setup_params() method.

Value

A gtable object.

attach_strips

Description

A function method that renders strips and attaches these to the gtable with panels and axes. The format_strip_labels() method is used to format the strip text. The default method returns the gtable unaltered.

Usage

Facet$attach_strips(table, layout, ranges, coord, theme, params)

Arguments

table: A gtable object from the attach_axes() method.
layout: A data frame computed by the compute_layout() method. Typically contains the faceting variables, ROW, COL, PANEL, SCALE_X and SCALE_Y variables.
params: A list of parameters coming from the setup_params() method.
theme: A complete theme object.

Value

A gtable object.

format_strip_labels

Description

A function method that formats the text for strips. It is used in the attach_strips methods, but also the get_strip_labels() function. The default method returns NULL.

Usage

Facet$format_strip_labels(layout, params)

Arguments

layout: A data frame computed by the compute_layout() method. Typically contains the faceting variables, ROW, COL, PANEL, SCALE_X and SCALE_Y variables.
params: A list of parameters coming from the setup_params() method.

Value

A list containing a data frame with strip labels.

set_panel_size

Description

A function method that enforces the panel.widths and panel.heights theme settings.

Usage

Facet$set_panel_size(table, theme)

Arguments

table: A gtable object populated by the draw_panels() method.
theme: A complete theme object.

Value

The table object, optionally with different widths and heights properties.

attach_axes

Description

A function method that renders axis titles and adds them to the gtable. The default is to add one title at each side depending on the position and presence of axes.

Usage

Facet$draw_labels(
  panels,
  layout,
  x_scales,
  y_scales,
  ranges,
  coord,
  data,
  theme,
  labels,
  params
)

Arguments

panels: A gtable object initiated by the draw_panels() method.
layout: A data frame computed by the compute_layout() method. Typically contains the faceting variables, ROW, COL, PANEL, SCALE_X and SCALE_Y variables.
x_scales,y_scales: A list of panel scales for each SCALE_X and SCALE_Y level respectively.
ranges: A list of panel parameters from the setup_panel_params() augmented with position guides.
coord: A ⁠<Coord>⁠ ggproto object.
data: A list of data frames containing layer data.
theme: A complete theme object.
labels: A named list containing an x list and y list. The x and y lists have primary and secondary labels.
params: A list of parameters coming from the setup_params() method.

Value

A gtable object.

vars

Description

A function method that returns the names of faceting variables. The default method returns an character vector with 0 length.

Usage

Facet$vars()

Value

A character vector

Conventions

The object name that a new class is assigned to is typically the same as the class name. Facet class names are in UpperCamelCase and start with the ⁠Facet*⁠ prefix, like FacetNew.

A constructor function is usually paired with a Facet class. The constructor copies the facet class and populates the params field. The constructor function name should take the Facet class name and be formatted with snake_case, so that FacetNew becomes facet_new().