How to add annotation on each facet

Question

I want to give each facet an alpha code, from A to H since there are eight facets, and draw each code on the top-left of each facet:

ggthemr('dust', layout = 'scientific',
        spacing = 1, type = 'inner', line_weight = 0.6,
        )

ptitles <- c('A' = "Total mass (g)", 'B' = "Root mass (g)", 'C' = "Stem mass (g)",
    'D' = "Leaf mass (g)", 'E' = "Number of nodes",
    'F' = "Number of leaves", 'G' = "Total stem length (cm)", 'H' = "RDI")

ggplot(gtr, aes(sediment, value)) +
    geom_boxplot(aes(fill = nitrogen)) +
    geom_text(aes(label = trait, group = trait)) +
    facet_wrap(~trait, scales = "free_y", ncol = 2,
        labeller = as_labeller(ptitles),
        strip.position = "left"
        ) +

theme(legend.position = "bottom",
    legend.title = element_text(size = 12),
    legend.key.size = unit(2, "lines"),
    legend.text = element_text(size = 12),
    strip.text.x = element_text(size = 12, margin = margin(0, 0, 0, 10)),
    strip.text.y = element_text(size = 14),
    strip.placement = "outside",
    axis.title.y = element_text(size = 14),
    axis.title.x = element_text(size = 14),
    axis.text.x = element_text(size = 14),
    panel.spacing.x = unit(0.5, "lines"),
    panel.spacing.y = unit(0.3, "lines"),
    aspect.ratio = 2 / 3
    ) +

xlab("Effects of sediment type and nitrogen deposition") +
ylab(NULL)

I tried to use geom_text():

geom_text(aes(label = trait, group = trait))

(Here the variable trait stores factors from A to H to distinguish each facet) But it did not work like what I expected:

Is there a simple way to such a thing?

UPDATE:

According to baptiste's answer, I changed my geom_text() code above to below:

    geom_text(aes(x = -Inf, y = Inf, label = trait, group = trait),
        size = 5,
        hjust = -0.5,
        vjust = 1.4,
        inherit.aes = FALSE)

inherit.aes = FALSE here seems to do nothing, how does this parameter work?.

Now my plot looks good:

score 14 · Accepted Answer · answered Sep 30 '17 at 04:33

library(ggplot2)

d <- data.frame(x=rep(1:3, 4), f=rep(letters[1:4], each=3))

labels <- data.frame(f=letters[1:4], label=LETTERS[1:4])
ggplot(d, aes(x,x)) +
  facet_wrap(~f) +
  geom_point() +
  geom_label(data = labels, aes(label=label), 
            x = Inf, y = -Inf, hjust=1, vjust=0,
            inherit.aes = FALSE)

How to add annotation on each facet

1 Answers1

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