Spider (Radar) Coordinate System for ggplot2

coord_spider() implements a new spider / radar coordinate system using a linear (non-munched) polar-like transform. It is intended for spider charts where connected paths are drawn as straight chords between axes (not circular arcs), and where background grid rings are polygons (not circles). Compared to ggplot2::coord_radial(), this coordinate system:

uses is_linear() = TRUE to prevent path "munching" into arcs;
supports a polygonal background grid (rings + spokes);
optionally draws axis labels (theta) and radial (r) labels using grid grobs in the panel background.

Typical usage is to map a discrete variable (categories) to x (theta) and a continuous variable (scores) to y (radius), then draw a polygon with geom_polygon().

plot2() uses this novel coordinate system to plot spider plots, created using plot2(..., type = "spider").

Usage

coord_spider(
  theta = "x",
  start = 0,
  clip = "off",
  inner.radius = 0,
  outer.radius = 0.8,
  axis_labels = TRUE,
  axis_label_size = 9,
  axis_label_pad_mm = 1,
  r_labels = TRUE,
  r_label_size = 8,
  r_label_pad_mm = 0
)

CoordSpider

Arguments

theta: Character scalar specifying which position aesthetic is interpreted as the angular coordinate (theta). Must be "x" or "y". In the common spider-chart case, use theta = "x" with a discrete x scale (categories around the circle) and a continuous y scale (radius). Setting theta = "y" swaps these roles.
start: Numeric scalar giving the angular offset in radians for the first axis. Angles increase according to the direction field stored in the coord (this implementation fixes direction = 1 internally). With start = 0, the first axis is at 12 o'clock (positive vertical). Use start = pi/2 to rotate the first axis to 3 o'clock, etc.
clip: Character scalar passed through to the coordinate system to control clipping of grobs to the panel. Use "on" to clip all drawing (including axis labels drawn in render_bg()) to the panel region, and "off" to allow grobs to extend outside the panel. When clip = "on", labels that lie beyond the panel boundary will be cut off, so in that case it is recommended to set outer.radius < 1 to reserve headroom inside the panel.
inner.radius: Numeric scalar in [0, 1) specifying the inner radius of the spider coordinate system as a fraction of the panel radius. Values greater than 0 create a "donut" / hollow center. The data radius is linearly rescaled onto [inner.radius, outer.radius].
outer.radius: Numeric scalar in (0, 1] specifying the maximum radius used for drawing the spider geometry as a fraction of the panel radius. Setting outer.radius < 1 shrinks the spider grid and data polygon inward, thereby reserving whitespace inside the panel for labels while keeping clip = "on". This is the preferred way to create consistent headroom independent of plot margins and device size.
axis_labels: Logical; if TRUE, draw the theta-axis labels (typically the discrete category labels) at the ends of the spokes using grid::textGrob in the panel background. Labels are taken from the trained discrete limits of the theta scale.
axis_label_size: Numeric scalar giving the font size (in points) for theta-axis labels drawn when axis_labels = TRUE.
axis_label_pad_mm: Numeric scalar giving the outward padding (in mm) applied to theta-axis labels beyond the spoke endpoints. This padding is applied in npc coordinates via grid::unit(). Increase this to separate labels from the grid. When clip = "on", excessive padding may push labels outside the panel and cause clipping; prefer reducing padding or decreasing outer.radius` in that case.
r_labels: Logical; if TRUE, draw radial (r / y-scale) labels along the "upper" spoke (the spoke at angle start, i.e. 12 o'clock when start = 0). Breaks and labels are taken from the trained radial scale (typically the y scale when theta = "x"). This means user-supplied scale_y_continuous(breaks = ...) will control both the grid rings and the printed radial labels.
r_label_size: Numeric scalar giving the font size (in points) for radial labels drawn when r_labels = TRUE.
r_label_pad_mm: Numeric scalar giving the horizontal padding (in mm) applied to radial labels from the upper spoke. Use positive values to move labels to the right of the spoke. With clip = "on", large padding can cause clipping; consider smaller padding and/or smaller outer.radius.

Value

A ggproto object that inherits from ggplot2::Coord. This object is used by ggplot2 to transform data coordinates and to render the panel background (grid rings/spokes and optional axis labels).

Examples

# Example data: one observation across five metrics.
# 'metric' is the discrete theta axis, 'value' is the radial axis.
library(ggplot2)

df <- data.frame(
  metric = c("Final exam grade",
             "Midterm grade",
             "Assignments completed",
             "Hours studying",
             "Classes attended"),
  value = c(70, 60, 10, 15, 25))
df
#>                  metric value
#> 1      Final exam grade    70
#> 2         Midterm grade    60
#> 3 Assignments completed    10
#> 4        Hours studying    15
#> 5      Classes attended    25

ggplot(df, aes(metric, value, group = 1)) +
    # Spider/Radar plots only require geom_polygon() with coord_spider():
    geom_polygon(colour = "red", fill = NA, linewidth = 1) +
    coord_spider()

    
ggplot(df, aes(metric, value, group = 1)) +
    geom_polygon(colour = "red", fill = NA, linewidth = 1) +
    geom_point(colour = "red", size = 3) +
    coord_spider(inner.radius = 0.25,
                 r_label_pad_mm = 2) +
    scale_y_continuous(limits = c(0, 100),
                       breaks = c(0, 25, 50, 75, 100)) +
    theme(panel.grid.major.y = element_line(colour = "blue"))

    
df |>
  plot2(type = "spider")
#> ℹ Using x = metric
#> ℹ Using y = value