ggplot2::Geom, ggplot2::Stat,
ggplot2::Position, or ggplot2::Scale.geom_* functions (like geom_point) return a layer that
contains a Geom* object (like GeomPoint). The Geom*
object is responsible for rendering the data in the plot.Each of the Geom* objects is a ggproto object, descended
from the top-level Geom, and each implements various methods and
fields. To create a new type of Geom object, you typically will want to
implement one or more of the following:
Compared to Stat and Position, Geom is a little
different because the execution of the setup and compute functions is
split up. setup_data runs before position adjustments, and
draw_layer is not run until render time, much later. This
means there is no setup_params because it's hard to communicate
the changes.
draw_panel(self, data, panel_scales, coord)ordraw_group(self, data, panel_scales, coord).draw_panelis
called once per panel,draw_groupis called once per group. Usedraw_panelif each row in the data represents a
single element. Usedraw_groupif each group represents
an element (e.g. a smooth, a violin).datais a data frame of scaled aesthetics.panel_scalesis a list containing information about the scales in the current
panel.coordis a coordinate specification. You'll
need to callcoord$transform(data, panel_scales)to work
with non-Cartesian coords. To work with non-linear coordinate systems,
you typically need to convert into a primitive geom (e.g. point, path
or polygon), and then pass on to the corresponding draw method
for munching.
Must return a grob. UsezeroGrobif there's nothing to
draw.
draw_key: Renders a single legend key.required_aes: A character vector of aesthetics needed to
render the geom.default_aes: A list (generated byaes()of
default values for aesthetics.reparameterise: Converts width and height to xmin and xmax,
and ymin and ymax values. It can potentially set other values as well.coord_* functions (like coord_trans) return a Coord*
object (like CoordTrans). The Coord* object is responsible for
adjusting the position of overlapping geoms.The way that the coord_* functions work is slightly different from the
geom_* and stat_* functions, because a coord_* function
actually "instantiates" the Coord* object by creating a descendant,
and returns that.
Each of the Coord* objects is a ggproto object,
descended from the top-level Coord. To create a new type of Coord
object, you typically will want to implement one or more of the following:
aspect: Returns the desired aspect ratio for the plot.labels: Returns a list containing labels for x and y.render_fg: Renders foreground elements.render_bg: Renders background elements.render_axis_h: Renders the horizontal axis.render_axis_v: Renders the vertical axis.range: Returns the x and y rangestrain: Return the trained scale ranges.transform: Transforms x and y coordinates.distance: Calculates distance.is_linear: ReturnsTRUEif the coordinate system is
linear;FALSEotherwise.stat_* functions (like stat_bin) return a layer that
contains a Stat* object (like StatBin). The Stat*
object is responsible for rendering the data in the plot.Each of the Stat* objects is a ggproto object, descended
from the top-level Stat, and each implements various methods and
fields. To create a new type of Stat object, you typically will want to
implement one or more of the following:
compute_layer(self, data, scales, ...),compute_panel(self, data, scales, ...), orcompute_group(self, data, scales, ...).compute_layer()is called once per layer,compute_panel_()is called once per panel, andcompute_group()is called once per
group. All must return a data frame. It's usually best to start by overridingcompute_group: if
you find substantial performance optimisations, override higher up.
You'll need to read the source code of the default methods to see
what else you should be doing.datais a data frame containing the variables named according
to the aesthetics that they're mapped to.scalesis a list
containing thexandyscales. There functions are called
before the facets are trained, so they are global scales, not local
to the individual panels....contains the parameters returned bysetup_params().
setup_params(data, params): called once for each layer.
Used to setup defaults that need to complete dataset, and to inform
the user of important choices. Should return list of parameters.setup_data(data, params): called once for each layer,
aftersetp_params(). Should return modifieddata.
Default methods removes all rows containing a missing value in
required aesthetics (with a warning if!na.rm).required_aes: A character vector of aesthetics needed to
render the geom.default_aes: A list (generated byaes()of
default values for aesthetics.position_* functions (like position_dodge) return a
Position* object (like PositionDodge). The Position*
object is responsible for adjusting the position of overlapping geoms.The way that the position_* functions work is slightly different from
the geom_* and stat_* functions, because a position_*
function actually "instantiates" the Position* object by creating a
descendant, and returns that.
Each of the Position* objects is a ggproto object,
descended from the top-level Position, and each implements the
following methods:
compute_layer(self, data, params, panel)is called once
per layer.panelis currently an internal data structure, so
this method should not be overriden.compute_panel(self, data, params, panel)is called once per
panel and should return a modified data frame.datais a data frame containing the variables named according
to the aesthetics that they're mapped to.scalesis a list
containing thexandyscales. There functions are called
before the facets are trained, so they are global scales, not local
to the individual panels.paramscontains the parameters returned bysetup_params().setup_params(data, params): called once for each layer.
Used to setup defaults that need to complete dataset, and to inform
the user of important choices. Should return list of parameters.setup_data(data, params): called once for each layer,
aftersetp_params(). Should return modifieddata.
Default checks that required aesthetics are present.And the following fields
required_aes: a character vector giving the aesthetics
that must be present for this position adjustment to work.scale_* functions (like scale_x_continuous) return a
Scale* object (like ScaleContinuous). The Scale*
object represents a single scale.Each of the Scale* objects is a ggproto object,
descended from the top-level Scale.