如何为ggplot2的facet_grid面板标题添加唯一triplet_id计数？

你的核心需求是给facet_grid中每个age_group对应的面板标题添加该组唯一triplet_id的数量，原代码的问题主要有两点：

1. 直接用length(unique(triplet_id))会计算整个数据集的唯一ID数，而非当前面板分组的数量；
2. x=5超出了x轴0-1的百分比范围，导致文本完全出现在绘图区域外，无法显示。

下面分两种场景给出解决方案：

场景1：将计数添加到面板标题中 #

先预处理数据计算每个age_group的唯一ID数量，再自定义标签替换原面板标题：

library(dplyr)
library(ggplot2)

# 计算每个age_group的唯一triplet_id数量
count_df <- graph %>%
  group_by(age_group) %>%
  summarise(unique_count = n_distinct(triplet_id)) %>%
  ungroup()

# 构建带计数的age_group标签
age_labs <- setNames(
  paste0(count_df$age_group, " (n=", count_df$unique_count, ")"),
  count_df$age_group
)

# 修正后的绘图代码
pl <- ggplot(data = graph,aes(y= Item, fill = Item_score))+ 
  geom_bar(stat="count", position = position_fill(reverse = TRUE), color = "black") + 
  scale_y_discrete(limits=rev)+ 
  scale_fill_manual(values = c("red", "white", "seagreen3"))+ 
  theme_classic() + 
  labs(y ="items", x = "Percentage")+ 
  scale_x_continuous(labels = scales::percent, breaks = seq(0, 1, .25))+ 
  # 使用自定义标签替换原age_group面板标题
  facet_grid(visit_type_1~age_group,  
             labeller = labeller(visit_type_1 = time.labs, age_group = age_labs))+ 
  theme(legend.position = "none",
        text = element_text(size = 30),
        strip.text.x = element_text(size = 30),
        strip.text.y = element_text(size = 30),
        panel.spacing.x=unit(1.5, "lines"),
        panel.spacing.y=unit(2, "lines"))+ 
  labs(title = "A")
pl

场景2：将计数添加到面板内部 #

如果需要在每个面板的绘图区域内显示计数，需先按facet的分组维度（visit_type_1+age_group）计算数量，再用geom_text添加：

library(dplyr)
library(ggplot2)

# 按面板分组计算唯一triplet_id数量
panel_counts <- graph %>%
  group_by(visit_type_1, age_group) %>%
  summarise(unique_count = n_distinct(triplet_id), .groups = "drop")

# 修正后的绘图代码
pl <- ggplot(data = graph,aes(y= Item, fill = Item_score))+ 
  geom_bar(stat="count", position = position_fill(reverse = TRUE), color = "black") + 
  scale_y_discrete(limits=rev)+ 
  scale_fill_manual(values = c("red", "white", "seagreen3"))+ 
  theme_classic() + 
  labs(y ="items", x = "Percentage")+ 
  scale_x_continuous(labels = scales::percent, breaks = seq(0, 1, .25))+ 
  facet_grid(visit_type_1~age_group,  labeller = labeller(visit_type_1 = time.labs))+ 
  theme(legend.position = "none",
        text = element_text(size = 30),
        strip.text.x = element_text(size = 30),
        strip.text.y = element_text(size = 30),
        panel.spacing.x=unit(1.5, "lines"),
        panel.spacing.y=unit(2, "lines"))+ 
  # 用预处理的分组计数数据添加文本，调整x/y到合适位置
  geom_text(data = panel_counts, aes(x = 0.5, y = 1, label = paste0("Count: ", unique_count)),
            inherit.aes = FALSE, size = 8)+ 
  labs(title = "A")
pl

内容的提问来源于stack exchange，提问作者Giorgia