patchwork: how to have within-panel tags for a 3x3 combined plot?

Similar to this problem <https://stackoverflow.com/questions/76524971/patchwork-align-tags-inside-panels-for-side-by-side-plots/76525099#76525099>, I need to move tags inside panel borders, but now for a 3x3 grid.

library(devtools)
library(ggplot2)
library(patchwork)

d1 <- runif(500)
d2 <- rep(c("Treatment","Control"),each=250)
d3 <- rbeta(500,shape1=100,shape2=3)
d4 <- d3 + rnorm(500,mean=0,sd=0.1)
plotData <- data.frame(d1,d2,d3,d4)
str(plotData)

This code is to make sure one combined legend is shown per row:

p1 <- ggplot(data=plotData) + geom_boxplot(aes(x=d2,y=d1,fill=d2)) + theme(legend.position = "none")
p2 <- ggplot(data=plotData) + geom_boxplot(aes(x=d2,y=d1,fill=d2)) + theme(legend.position = "none")
p3 <- ggplot(data=plotData) + geom_boxplot(aes(x=d2,y=d1,fill=d2)) + theme(legend.position =  "right")
p4 <- ggplot(data=plotData) + geom_boxplot(aes(x=d2,y=d1,fill=d2)) + theme(legend.position = "none")
p5 <- ggplot(data=plotData) + geom_boxplot(aes(x=d2,y=d1,fill=d2)) + theme(legend.position = "none")
p6 <- ggplot(data=plotData) + geom_boxplot(aes(x=d2,y=d1,fill=d2)) + theme(legend.position =  "right")
p7 <- ggplot(data=plotData) + geom_boxplot(aes(x=d2,y=d1,fill=d2)) + theme(legend.position = "none")
p8 <- ggplot(data=plotData) + geom_boxplot(aes(x=d2,y=d1,fill=d2)) + theme(legend.position = "none")
p9 <- ggplot(data=plotData) + geom_boxplot(aes(x=d2,y=d1,fill=d2)) + theme(legend.position = "right")

This code produced a combined plot with tags, at default locations

(((p1 + p2 + p3) / 
(p4 + p5 + p6) / 
(p7 + p8 + p9)) + guide_area()) & plot_annotation(tag_levels = "A") 

Adding & plot_annotation(tag_levels = "A") + theme(legend.position = "right", plot.tag.position = c(.15, .95)) removed the tags.

Thanks for stopping by.

Perhaps there is a more straightforward approach to achieve your desired result but one option would be to fake your inner tags using annotate or annotation_custom as I do below which means to manually add the "tags" as annotations to each plot:

library(ggplot2)
library(patchwork)

plot_list <- lapply(1:9, \(x) {
  legend_position <- if (x %% 3 == 0) {
    "right"
  } else {
    "none"
  }
  ggplot(data = plotData) +
    geom_boxplot(aes(x = d2, y = d1, fill = d2)) +
    theme(legend.position = legend_position)
})

add_tag <- function(label, x = 1, y = 1, padding.x = unit(5, "pt"), padding.y = padding.x, hjust = 1, vjust = 1, size = 13) {
  annotation_custom(
    grid::textGrob(
      x = unit(x, "npc") - padding.x,
      y = unit(y, "npc") - padding.y,
      hjust = hjust, vjust = vjust,
      label = label, gp = grid::gpar(fontsize = size)
    )
  )
}

# Default: topright corner
lapply(
  seq_along(plot_list), \(x) plot_list[[x]] + add_tag(LETTERS[x])
) |>
  wrap_plots(ncol = 3)

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# Tags in "topleft" corner
lapply(
  seq_along(plot_list), \(x) plot_list[[x]] + add_tag(
    LETTERS[x],
    x = 0, y = 1,
    hjust = 0, vjust = 1,
    padding.x = -unit(5, "pt"), padding.y = unit(5, "pt")
  )
) |>
  wrap_plots(ncol = 3)

<!-- -->

Many thanks to Stefan for the hints. This code handles unidentical subplots.

# List the subplots
plot_list <- list(p1, p2, p3,p4, p5, p6, p7, p8, p9)

# Define a function to add a tag to a plot
add_tag <- function(label, x = 1, y = 1, padding.x = unit(5, "pt"), padding.y = padding.x, hjust = 1, vjust = 1, size = 13) {
  annotation_custom(
    grid::textGrob(
      x = unit(x, "npc") - padding.x,
      y = unit(y, "npc") - padding.y,
      hjust = hjust, vjust = vjust,
      label = label, gp = grid::gpar(fontsize = size)
    )
  )
}

# Add a tag to each plot
plot_list <- lapply(
  seq_along(plot_list), 
  \(x) plot_list[[x]] + add_tag(LETTERS[x], x = .15, y = .96)
)

# Combine the plots into a grid
wrap_plots(plot_list, ncol = 3)