Biplot of a two-dimensional projection — biplot

Draw a two-dimensional projection of the scores with the original variable axes overlaid as arrows (loadings), as in a biplot. Pass object (the output of stats::prcomp() or an rrcov::Pca* function); otherwise supply scores and loadings directly. All scores should be centred about the origin. The arrows are stretched by a common factor so that the longest arrow just reaches the edge of the point cloud, and only loadings longer than label_threshold are labelled.

Usage

biplot_projection(
  object = NULL,
  scores = NULL,
  loadings = NULL,
  label_threshold = 0,
  arrow_colour = "#c14b14",
  ...
)

Arguments

object: Optionally, the output of stats::prcomp() or an rrcov::Pca* function. If supplied, the scores and loadings are extracted from it and the scores and loadings arguments are ignored.
scores: A matrix or data frame of scores centred about the origin, with the first two columns used as the horizontal and vertical coordinates. Ignored if object is supplied.
loadings: A matrix or data frame of loadings, with row names giving the variable names and the first two columns used as the arrow directions. Ignored if object is supplied.
label_threshold: Only loadings whose absolute length exceeds this threshold are labelled. The default of 0 labels every non-zero loading.
arrow_colour: Colour of the arrows and labels.
...: Additional arguments passed to ggplot2::geom_point().

Value

A ggplot object.

References

Hyndman, R J (2026) "That's weird: Anomaly detection using R", Chapter 9, https://OTexts.com/weird/.

Author

Rob J Hyndman

Examples

oldfaithful[, c("duration", "waiting")] |>
  prcomp(scale = TRUE) |>
  biplot_projection()