Add iso-contour lines to a texture triangle plot — geom_texture

Interpolates a numeric variable from scattered ternary sample points onto a regular Cartesian grid (using IDW) and draws contour lines at the specified values. Add to a plot produced by gg_texture_triangle() with +.

Usage

geom_texture_contour(
  data,
  sand,
  silt,
  clay,
  z,
  breaks = NULL,
  resolution = 150L,
  power = 2,
  ...
)

Arguments

data: A data frame containing sample points.
sand, silt, clay: Bare column names for the ternary coordinates (0–100).
z: Bare column name for the numeric value to interpolate.
breaks: Numeric vector of z values at which to draw contour lines. Passed to ggplot2::geom_contour(). If NULL (default), ggplot2 chooses breaks automatically.
resolution: Integer. Interpolation grid width (default 150L).
power: Positive numeric. IDW exponent (default 2).
...: Additional arguments passed to ggplot2::geom_contour() (e.g. linewidth, colour, linetype).

Value

A geom_contour ggplot2 layer.

Details

Contour lines are rendered on top of all triangle layers, making them suitable for highlighting analytical thresholds (e.g. a significance boundary at p = 0.05).

Examples

set.seed(7)
surf_df <- data.frame(
  sand = runif(80, 5, 90),
  clay = runif(80, 5, 60)
) |>
  dplyr::mutate(silt = 100 - sand - clay) |>
  dplyr::filter(silt >= 0) |>
  dplyr::mutate(p_val = pnorm(scale(clay)[, 1]))

pts <- data.frame(sand = c(40, 20), silt = c(40, 30), clay = c(20, 50))

gg_texture_triangle(pts, sand, silt, clay) +
  geom_texture_contour(surf_df, sand, silt, clay, z = p_val,
                       breaks = 0.05, linewidth = 1.4, colour = "red")
#> Warning: `stat_contour()`: Zero contours were generated
#> Warning: no non-missing arguments to min; returning Inf
#> Warning: no non-missing arguments to max; returning -Inf