Create Compact Letter Display Layer Groups with at least one letter in common are not significantly different.

Usage

geom_tukey(
  test = c("tukey", "kruskalmc", "dunn"),
  type = c("two-way", "one-way"),
  threshold = 0.05,
  where = c("box", "whisker", "mean", "median", "se", "sd", "cl_normal", "cl_boot"),
  hjust = 0,
  vjust = -0.2,
  geom = "text",
  size = 4,
  color = "black",
  fill = "white",
  alpha = 1,
  na.rm = TRUE,
  reversed = FALSE,
  Letters = letters
)

Arguments

test: Which test to run for pairwise comparisons. Either tukey (the default), kruskalmc, or dunn_test.
type: If a grouping variable is provided, determines whether to run separate tests for each facet (one-way) or one (two-way) test with an interaction term between x and group. Defaults to two-way.
threshold: Statistical threshold for significance. Defaults to 0.05.
where: Where to put the letters. Either above the box (box) or upper whisker (whisker) of a boxplot; at the mean or median of the distribution; at the top of the error bars calculated from the standard error (se), standard deviation sd, or 95% confidence intervals returned by Hmisc::smean.cl.normal() (cl_normal), or Hmisc::smean.cl.boot() (cl_boot); or a fixed numeric y-position. A single number is recycled across all groups; a vector must have one value per group/facet, matching sorted group order.
hjust: Horizontal adjustment of the label. (Argument to geom_text).
vjust: Vertical adjustment of the label. (Argument to geom_text).
geom: Which geom to use to plot letters. Options are text and label.
size: Label size. Argument to geom_text.
color: Label color.
fill: Label fill (only applies if geom == "label").
alpha: Label transparency. Defaults to 1.
na.rm: Logical. Whether to remove observations with NAs for the provided factors (i.e. x and group) before plotting. Defaults to TRUE.
reversed: Logical. Argument to multcompLetters3. Determines whether order of letters should be reversed. Defaults to FALSE.
Letters: Character vector of symbols used to form the compact letter display. Defaults to letters (a, b, c, …). Pass LETTERS for uppercase, or any character vector for custom symbols (e.g. Greek letters or Unicode).

Note

Thank you to Hiroaki Yutani and Simon P. Couch for a couple of very helpful blog posts (1, 2) describing the ggplot_add syntax.

References

Piepho, Hans-Peter. An Algorithm for a Letter-Based Representation of All-Pairwise Comparisons. Journal of Computational and Graphical Statistics 13, no. 2 (June 1, 2004): 456–66. doi:10.1198/1061860043515 .
Piepho, Hans-Peter. "Letters in Mean Comparisons: What They Do and Don't Mean." Agronomy Journal 110, no. 2 (2018): 431–34. doi:10.2134/agronj2017.10.0580
Graves S, Piepho H, Dorai-Raj LSwhfS (2019). multcompView: Visualizations of Paired Comparisons. R package version 0.1-8. https://CRAN.R-project.org/package=multcompView

Author

Ethan Bass

Examples

library(ggplot2)
set.seed(1)
data <- data.frame("Category" = c(rep("Low", 10), rep("Medium", 10), rep("High", 10)),
                  "Value" = c(rnorm(10, 5), rnorm(10, 5.5), rnorm(10, 10)),
                  "Size" = c("Big","Small"))
data |> ggplot(aes(x=Category, y=Value)) + geom_boxplot() + facet_wrap(~Size) + geom_tukey()

data |> ggplot(aes(x=Size, y=Value)) + geom_boxplot() + facet_wrap(~Category) + geom_tukey()