Background

I am writing a ggplot2 extension and wish to make use of ggplot_add() with a custom method so that I can influence behavior down the ggplot chain while referencing information from up the ggplot chain, such as attributes and layer position.

I chose to use ggplot_add() because I wanted to influence how labels appear in the legend of the ggplot output using guides() in combination with override.aes.

Reprex

Below is a reprex with a geom function to establish the structure, a custom ggplot_add() method to create the layer and influence behavior, and a custom Geom built almost completely after GeomLabel.

Note I am not supplying custom_legend_fixing_function(), as it is just a placeholder example.

library(ggplot2)

geom_new_label <- function(mapping = NULL, data = NULL,
                            stat = "identity", position = "identity",
                            ...,
                            parse = FALSE,
                            label.padding = unit(0.25, "lines"),
                            label.r = unit(0.15, "lines"),
                            label.size = 0.25,
                            size.unit = "mm",
                            na.rm = FALSE,
                            show.legend = NA,
                            inherit.aes = TRUE) {
  structure(
    "geom_new_label",
    class = c("new_label", "ggnew_layer"),
    stat = stat,
    position = position,
    mapping = mapping,
    data = data,
    show.legend = show.legend,
    inherit.aes = inherit.aes,
    params = list(
      na.rm = na.rm,
      parse = parse,
      label.padding = label.padding,
      label.r = label.r,
      label.size = label.size,
      size.unit = size.unit,
      ... = ...
    )
  )
}

ggplot_add.new_label <- function(object, plot, object_name) {

  new_layer <- layer(
    data = attr(object, "data"),
    mapping = attr(object, "mapping"),
    stat = attr(object, "stat"),
    geom = GeomNewLabel,
    position = attr(object, "position"),
    key_glyph = "label",
    show.legend = attr(object, "show.legend"),
    inherit.aes = attr(object, "inherit.aes"),
    params = attr(object, "params")
  )
  
  plot$layers <- append(plot$layers, new_layer)
  
  # Fix legend ----
  # Impetus for using ggplot_add(), function logic not shown
  label_override <- custom_legend_fixing_function(plot, new_layer)
  
  plot <- plot +
    guides(
      colour = guide_legend(
        override.aes = list(
          label = label_override
        )
      )
    )
  
  plot
}

GeomNewLabel <- ggproto("GeomNewLabel", Geom,
                         required_aes = c("x", "y", "label"),
                         default_aes = aes(
                           colour = NA, fill = NA, size = 3.88, angle = 0,
                           hjust = 0.5, vjust = 0.5, alpha = NA, family = "", fontface = 1,
                           lineheight = 1.2
                         ),
                         draw_panel = function(data, panel_params, coord, parse = FALSE,
                                               na.rm = FALSE,
                                               label.padding = unit(0.25, "lines"),
                                               label.r = unit(0.15, "lines"),
                                               label.size = 0.25,
                                               size.unit = "mm") {
                           
                           # Return all components
                           grid::gList(
                             GeomLabel$draw_panel(data, panel_params, coord,
                                                  parse = FALSE,
                                                  na.rm = na.rm,
                                                  label.padding = label.padding,
                                                  label.r = label.r,
                                                  label.size = label.size,
                                                  size.unit = size.unit
                             )
                           )
                         }
)

^{Created on 2024-06-12 with reprex v2.1.0}

The Problem

For typical warnings, like below, the parent reference points to the original function:

mtcars |> 
  ggplot() +
  geom_label(mapping = aes(x = mpg, y = cyl, label = disp),
             test = "test")

Warning message:
In geom_label(mapping = aes(x = mpg, y = cyl, label = disp), test = "test") :
  Ignoring unknown parameters: `test`

However, with my use of ggplot_add() it references ggplot_add():

Warning message:
In ggplot_add(object, p, objectname) :
  Ignoring unknown parameters: `test`

I realize this is expected, and because the call to layer() occurs inside of ggplot_add(), but I’m wondering if there is a way I can restructure my setup so it comes from geom_new_label()?

Thank you!