geom_arrowsegment {ggarchery}

Line segments with flexible arrows

Description

The basic geom_arrowsegment() is equivalent to geom_segment(arrow = arrow()). (It is assumed that the user wants some kind of arrow.) The extended functionality is to allow free placement of the arrowhead anywhere along the segment, and also multiple arrowheads, and to allow a fill aesthetic (which will only be visible for closed arrowheads).

The function works by dividing the line up into 1 or more segment grobs, each of which is generated by grid::arrow() except potentially the last (the one closest to the point (xend, yend)). The vector arrow_positions, whose entries must lie between 0 and 1, defines where each arrow segment ends, as a proportional position along the line. If the last entry of arrow_positions is 1, then the last grob has an arrow; otherwise it does not.

The function is designed with the expectation that arrows point from (x, y) to (xend, yend) but the arrows argument will happily accept arrow(ends = "first") or arrow(ends = "both") if you prefer. Just remember that the final segment is only an arrow at all if the last entry of arrow_positions is 1.

Usage

geom_arrowsegment(
  mapping = NULL,
  data = NULL,
  stat = "identity",
  position = "identity",
  ...,
  arrows = list(arrow()),
  arrow_fills = NULL,
  arrow_positions = 1,
  lineend = "butt",
  linejoin = "round",
  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

The statistical transformation to use on the data for this layer, either as a ggproto Geom subclass or as a string naming the stat stripped of the stat_ prefix (e.g. "count" rather than "stat_count")

position

Position adjustment, either as a string naming the adjustment (e.g. "jitter" to use position_jitter), or the result of a call to a position adjustment function. Use the latter if you need to change the settings of the adjustment.

...

Other arguments passed on to layer(). These are often aesthetics, used to set an aesthetic to a fixed value, like colour = "red" or size = 3. They may also be parameters to the paired geom/stat.

arrows

Either an arrow generated by grid::arrow() of a list of such arrows. In the former case or if the list has length 1, the arrowhead so defined is used every time; otherwise the list is expected to have the same length as arrow_positions and each segment defined by that argument is ended by the respective element of this one. The default is grid::arrow() with default parameters.

arrow_fills

A vector of fill colours for the arrowheads, behaves as the arrow_fill option in geom_segment. This will overrule a fill aesthetic in the same way that specifying a single fill outside aes specification will.

arrow_positions

A vector of distinct points on the unit interval. 0 is not permitted but arbitrarily small values are; 1 is permitted. The default behaviour is that arrowheads will be placed proportionally along the line connecting (x, y) to (xend,yend) at these points. In more detail: The first arrow segment begins at (x, y) and ends a proportional distance along the straight line joining (x, y) and (xend, yend) equal to the first entry of this vector. The second bridges the first two entries, and so on. If the final entry is 1 then the last segment is an arrow (and hence usually an arrowhead will be placed at the end of the line). If it is not, then the last segment is simply a line. These will be sorted into order from 0 to 1 if they are not already.

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.

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().

Value

A ggproto object

Examples

 library(ggplot2)
 library(magrittr)
 library(tidyr)

 # Generate some dummy data

 ten.points <- data.frame(line.no = rep(1:5, each = 2), x = runif(10), y = runif(10),
                          position = rep(c("start", "end"), 5))
 five.segments <- ten.points %>% pivot_wider(names_from = position, values_from = c(x,y))

 # Default behaviour

 ggplot(five.segments) +
    geom_point(data = ten.points, aes(x = x, y = y)) +
    geom_arrowsegment(aes(x = x_start, xend = x_end, y = y_start, yend = y_end))

 # Midpoint arrowheads

 ggplot(five.segments) +
    geom_point(data = ten.points, aes(x = x, y = y)) +
    geom_arrowsegment(aes(x = x_start, xend = x_end, y = y_start, yend = y_end),
                      arrow_positions = 0.5)

 # Double arrows

 ggplot(five.segments) +
    geom_point(data = ten.points, aes(x = x, y = y)) +
    geom_arrowsegment(aes(x = x_start, xend = x_end, y = y_start, yend = y_end),
                      arrow_positions = c(0.25, 0.75))

 # Double arrows, last arrowhead at the end point

 ggplot(five.segments) +
    geom_point(data = ten.points, aes(x = x, y = y)) +
    geom_arrowsegment(aes(x = x_start, xend = x_end, y = y_start, yend = y_end),
                      arrow_positions = c(0.25, 1))

 # Double arrowheads of varying appearance and position

 ggplot(five.segments) +
     geom_point(data = ten.points, aes(x = x, y = y)) +
     geom_arrowsegment(aes(x = x_start, xend = x_end, y = y_start, yend = y_end),
                       arrow_positions = c(0.25, 0.75),
                       arrows = list(arrow(angle = 45, type = "closed"),
                                     arrow(angle = 25, ends = "both")),
                       arrow_fills = "indianred")