如何为分组ggplot组合图添加统一图例及分组标题？

可复现数据集与基础绘图代码 #

先构造模拟数据集，再生成4个需要组合的子图：

library(ggplot2)
library(patchwork)
library(dplyr)

# 构造可复现数据集
set.seed(123)
data <- mtcars %>%
  mutate(
    sex = rep(c("Men", "Women"), each = 16),
    subgroup = rep(c("Subplot 1", "Subplot 2"), times = 16),
    value = rnorm(nrow(mtcars), mean = mpg, sd = 2)
  )

# 生成4个子图，隐藏单个图例避免重复
p_men1 <- ggplot(data %>% filter(sex == "Men", subgroup == "Subplot 1"), 
                 aes(x = wt, y = value, color = factor(cyl))) +
  geom_point() + theme_bw() + theme(legend.position = "none")

p_men2 <- ggplot(data %>% filter(sex == "Men", subgroup == "Subplot 2"), 
                 aes(x = wt, y = value, color = factor(cyl))) +
  geom_point() + theme_bw() + theme(legend.position = "none")

p_women1 <- ggplot(data %>% filter(sex == "Women", subgroup == "Subplot 1"), 
                   aes(x = wt, y = value, color = factor(cyl))) +
  geom_point() + theme_bw() + theme(legend.position = "none")

p_women2 <- ggplot(data %>% filter(sex == "Women", subgroup == "Subplot 2"), 
                   aes(x = wt, y = value, color = factor(cyl))) +
  geom_point() + theme_bw() + theme(legend.position = "none")

最优解决方案：使用patchwork包 #

patchwork能同时实现分组标题添加和统一图例收集，且不会挤压子图：

# 组合男女分组子图并添加分组标题
men_group <- p_men1 + p_men2 + 
  plot_annotation(title = "Men", 
                  theme = theme(plot.title = element_text(hjust = 0.5, size = 14, face = "bold")))

women_group <- p_women1 + p_women2 + 
  plot_annotation(title = "Women", 
                  theme = theme(plot.title = element_text(hjust = 0.5, size = 14, face = "bold")))

# 整体布局：上下排列分组，自动收集统一图例
final_plot <- (men_group / women_group) + 
  plot_layout(guides = "collect") + # 自动合并所有子图的共同图例
  plot_annotation(theme = theme(plot.margin = margin(10, 10, 10, 10))) # 微调边距避免图例拥挤

print(final_plot)

代码说明 #

1. 隐藏单个图例：每个子图设置theme(legend.position = "none")，避免重复显示图例。
2. 分组标题：用plot_annotation给男女分组的子图组合添加标题，并居中对齐。
3. 统一图例：plot_layout(guides = "collect")会自动提取所有子图的共同图例，默认放置在右侧，不会挤压绘图区域。
4. 边距调整：通过plot.margin微调整体边距，让布局更美观。

备选方案：手动提取图例并放置 #

如果需要更灵活的图例位置，可手动提取图例后嵌入布局：

# 提取统一图例
shared_legend <- get_legend(
  ggplot(data, aes(x = wt, y = value, color = factor(cyl))) +
    geom_point() + theme_bw() +
    guides(color = guide_legend(title = "Number of Cylinders"))
)

# 组合布局：左右分栏，左侧放分组图，右侧放图例
final_plot_custom <- (men_group / women_group) + shared_legend +
  plot_layout(widths = c(4, 1)) # 控制绘图区与图例区的宽度比例

print(final_plot_custom)

内容的提问来源于stack exchange，提问作者Vicki Latham