Merging of `guide_legend_group`s raises an error

[trekonom]

Hi Teun,
when updating some older answers on SO to account for legendry (e.g. this one) I came across an issue where merging of guide_legend_groups fails and raises an error. Here is a minimal reprex of the issue:

library(ggplot2)
library(legendry)

mtcars2 <- mtcars
mtcars2$cyl_am <- paste0(mtcars$cyl, ".", mtcars$am)
ggplot(mtcars2, aes(mpg, hp)) +
  geom_point(
    aes(
      shape = cyl_am, color = cyl_am
    )
  ) +
  guides(
    color = "legend_group",
    shape = "legend_group"
  )
#> New names:
#> • `.group` -> `.group...4`
#> • `.group` -> `.group...6`
#> Error in `vec_slice()`:
#> ! `x` must be a vector, not `NULL`.

^{Created on 2025-12-28 with reprex v2.1.1}

Session info

sessioninfo::session_info()
#> ─ Session info ───────────────────────────────────────────────────────────────
#>  setting  value
#>  version  R version 4.5.2 (2025-10-31)
#>  os       macOS Sequoia 15.6.1
#>  system   aarch64, darwin20
#>  ui       X11
#>  language (EN)
#>  collate  en_US.UTF-8
#>  ctype    en_US.UTF-8
#>  tz       Europe/Berlin
#>  date     2025-12-28
#>  pandoc   3.6.3 @ /Applications/RStudio.app/Contents/Resources/app/quarto/bin/tools/aarch64/ (via rmarkdown)
#>  quarto   1.8.24 @ /Applications/quarto/bin/quarto
#> 
#> ─ Packages ───────────────────────────────────────────────────────────────────
#>  package      * version date (UTC) lib source
#>  cli            3.6.5   2025-04-23 [1] CRAN (R 4.5.0)
#>  digest         0.6.39  2025-11-19 [1] CRAN (R 4.5.2)
#>  dplyr          1.1.4   2023-11-17 [1] CRAN (R 4.5.0)
#>  evaluate       1.0.5   2025-08-27 [1] CRAN (R 4.5.0)
#>  farver         2.1.2   2024-05-13 [1] CRAN (R 4.5.0)
#>  fastmap        1.2.0   2024-05-15 [1] CRAN (R 4.5.0)
#>  fs             1.6.6   2025-04-12 [1] CRAN (R 4.5.0)
#>  generics       0.1.4   2025-05-09 [1] CRAN (R 4.5.0)
#>  ggplot2      * 4.0.1   2025-11-14 [1] CRAN (R 4.5.2)
#>  glue           1.8.0   2024-09-30 [1] CRAN (R 4.5.0)
#>  gtable         0.3.6   2024-10-25 [1] CRAN (R 4.5.0)
#>  htmltools      0.5.9   2025-12-04 [1] CRAN (R 4.5.2)
#>  knitr          1.51    2025-12-20 [1] CRAN (R 4.5.2)
#>  labeling       0.4.3   2023-08-29 [1] CRAN (R 4.5.0)
#>  legendry     * 0.2.4   2025-09-14 [1] CRAN (R 4.5.0)
#>  lifecycle      1.0.4   2023-11-07 [1] CRAN (R 4.5.0)
#>  magrittr       2.0.4   2025-09-12 [1] CRAN (R 4.5.0)
#>  otel           0.2.0   2025-08-29 [1] CRAN (R 4.5.0)
#>  pillar         1.11.1  2025-09-17 [1] CRAN (R 4.5.0)
#>  pkgconfig      2.0.3   2019-09-22 [1] CRAN (R 4.5.0)
#>  R6             2.6.1   2025-02-15 [1] CRAN (R 4.5.0)
#>  RColorBrewer   1.1-3   2022-04-03 [1] CRAN (R 4.5.0)
#>  reprex         2.1.1   2024-07-06 [1] CRAN (R 4.5.0)
#>  rlang          1.1.6   2025-04-11 [1] CRAN (R 4.5.0)
#>  rmarkdown      2.30    2025-09-28 [1] CRAN (R 4.5.0)
#>  rstudioapi     0.17.1  2024-10-22 [1] CRAN (R 4.5.0)
#>  S7             0.2.1   2025-11-14 [1] CRAN (R 4.5.2)
#>  scales         1.4.0   2025-04-24 [1] CRAN (R 4.5.0)
#>  sessioninfo    1.2.3   2025-02-05 [1] CRAN (R 4.5.0)
#>  tibble         3.3.0   2025-06-08 [1] CRAN (R 4.5.0)
#>  tidyselect     1.2.1   2024-03-11 [1] CRAN (R 4.5.0)
#>  vctrs          0.6.5   2023-12-01 [1] CRAN (R 4.5.0)
#>  withr          3.0.2   2024-10-28 [1] CRAN (R 4.5.0)
#>  xfun           0.55    2025-12-16 [1] CRAN (R 4.5.2)
#>  yaml           2.3.12  2025-12-10 [1] CRAN (R 4.5.2)
#> 
#>  [1] /Users/stefan.moog/Library/R/arm64/4.5/library
#>  [2] /Library/Frameworks/R.framework/Versions/4.5-arm64/Resources/library
#>  * ── Packages attached to the search path.
#> 
#> ──────────────────────────────────────────────────────────────────────────────

On closer inspection, I found that the issue can be traced back to the merge method of GuideLegend, which does not account for the .group column. A possible fix would therefore be to add a custom merge method that accounts for the .group column, e.g. something like:

library(ggplot2)
library(legendry)
library(vctrs)

guide_legend_group2 <- function(
    key = "group_split",
    title = waiver(),
    override.aes = list(),
    nrow = NULL,
    ncol = NULL,
    theme = NULL,
    position = NULL,
    direction = NULL,
    order = 0
) {
  
  legendry:::check_position(position, theta = FALSE, inside = TRUE, allow_null = TRUE)
  legendry:::check_argmatch(direction, c("horizontal", "vertical"), allow_null = TRUE)
  legendry:::check_number_whole(nrow, min = 1, allow_null = TRUE)
  legendry:::check_number_whole(ncol, min = 1, allow_null = TRUE)
  legendry:::check_exclusive(nrow, ncol)
  
  new_guide(
    key = key,
    title = title,
    theme = theme,
    override.aes = legendry:::rename_aes(override.aes),
    nrow = nrow,
    ncol = ncol,
    order = order,
    available_aes = "any",
    name = "legend_group2",
    direction = direction,
    position = position,
    super = GuideLegendGroup2
  )
}

GuideLegendGroup2 <- ggproto(
  "GuideLegendGroup2", GuideLegendGroup,
  merge = function(self, params, new_guide, new_params) {
    # Combine keys
    
    #### Drop the .group column
    new_params$key$.group <- new_params$key$.label <- new_params$key$.value <- NULL
    ####
    
    params$key <- vec_cbind(params$key, new_params$key)
    
    # Combine override.aes
    params$override.aes <- c(params$override.aes, new_params$override.aes)
    nms <- names(params$override.aes)
    if (anyDuplicated(nms)) {
      cli::cli_warn("Duplicated {.arg override.aes} is ignored.")
    }
    params$override.aes <- params$override.aes[!duplicated(nms)]
    params$aesthetic <- union(params$aesthetic, new_params$aesthetic)
    
    list(guide = self, params = params)
  }
)

mtcars2 <- mtcars
mtcars2$cyl_am <- paste0(mtcars$cyl, ".", mtcars$am)
ggplot(mtcars2, aes(mpg, hp)) +
  geom_point(
    aes(
      shape = cyl_am, color = cyl_am
    )
  ) +
  guides(
    color = "legend_group2",
    shape = "legend_group2"
  )

^{Created on 2025-12-28 with reprex v2.1.1}