如何将共享图例置于2×2绘图网格的右下角空位？

要实现把共用图例放在2×2网格的右下角空位，不需要额外的包，用ggplot2和patchwork就能搞定，核心思路是单独提取图例，把它当作第四块图来布局，而不是用plot_spacer()占位。

具体修改步骤 #

1. 隐藏所有单个子图的图例，避免重复显示
2. 从任意一个带图例规则的图中提取共用图例对象
3. 将提取的图例作为第四个组件，和三个子图按2×2结构组合

修改后的完整代码 #

library(ggplot2)
library(patchwork)

set.seed(123)
dummy_data <- data.frame(
 month = rep(c(0, 6, 18, 30), 4),
 value = c(100, 70, 50, 30,
           100, 75, 55, 35,
           100, 65, 45, 25,
           100, 80, 60, 40),
 treatment = rep(c("true", "false"), each = 8),
 group = rep(c("A", "B"), each = 4, times = 2)
)

a <- ggplot(dummy_data[dummy_data$group == "A", ], 
            aes(x = month, y = value, color = treatment, group = treatment)) +
 geom_line(size = 1) + geom_point(size = 2) +
 theme_minimal() +
 labs(title = "Plot A (Group A)") +
 theme(legend.position = "none")  # 隐藏单个图的图例

b <- ggplot(dummy_data[dummy_data$group == "B", ], 
            aes(x = month, y = value, color = treatment, group = treatment)) +
 geom_line(size = 1) + geom_point(size = 2) +
 theme_minimal() +
 labs(title = "Plot B (Group B)") +
 theme(legend.position = "none")  # 隐藏单个图的图例

c <- ggplot(dummy_data, 
            aes(x = month, y = value, color = treatment, group = interaction(treatment, group))) +
 geom_line(size = 1) + geom_point(size = 2) +
 theme_minimal() +
 labs(title = "Plot C (All Groups)") +
 theme(legend.position = "none")  # 隐藏单个图的图例

# 自定义函数提取图例对象
get_legend <- function(my_plot) {
  tmp <- ggplot_gtable(ggplot_build(my_plot))
  leg_index <- which(sapply(tmp$grobs, function(x) x$name) == "guide-box")
  legend <- tmp$grobs[[leg_index]]
  return(legend)
}

# 生成临时图提取共用图例
tmp_plot <- ggplot(dummy_data, aes(x = month, y = value, color = treatment)) + geom_line()
shared_legend <- get_legend(tmp_plot)

# 按2×2布局组合图与图例
combined_plot <- (a + b) / (c + shared_legend) +
  plot_annotation(tag_levels = "A")

combined_plot

关键说明 #

• 给每个子图添加theme(legend.position = "none")，确保子图单独显示时不出现图例
• 通过get_legend()函数从ggplot底层对象中提取图例组件，保证图例样式与原图一致
• 布局时直接将图例替换原plot_spacer()的位置，即可让图例填充右下角的空位

内容的提问来源于stack exchange，提问作者Gabriel