如何正确合并两个ggplot并对齐坐标轴、分条与标题？

核心问题与修复思路 #

• 标签对齐问题：(A)(B)是分面的strip文本，(C)是绘图标题，两者布局逻辑不同，需统一为分面标签实现对齐
• y轴对齐问题：需统一两个图的y轴范围、纵横比，并在合并时启用垂直对齐

修复后的完整代码 #

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

# 第一个分面图：修正cyl数值判断，统一y轴范围
p1 <- mtcars %>%
  filter(cyl %in% c(4, 6)) %>%
  mutate(cyl_label = paste0("label_", cyl)) %>%
  ggplot(aes(x = mpg, y = wt)) +
  geom_point() +
  theme_bw() +
  theme(aspect.ratio = 2,
        strip.text.x = element_text(hjust = 0, colour = "black", size = 14,
                                    margin = unit(c(t=0.15,r=0,b=0.15,l=0), "cm")),
        strip.background = element_blank()) +
  facet_wrap(~ cyl_label, 
             labeller = labeller(cyl_label = c(label_4 = "(A) First plot",
                                               label_6 = "(B) Second plot"))) +
  coord_cartesian(ylim = range(mtcars$wt)) # 锁定y轴范围

# 第二个图：用单分面替代标题，匹配p1的标签位置与y轴设置
p2 <- mtcars %>%
  ggplot(aes(x = hp, y = wt)) +
  geom_point() +
  theme_bw() +
  theme(aspect.ratio = 2, # 统一纵横比，保证y轴视觉长度一致
        strip.text.x = element_text(hjust = 0, colour = "black", size = 14,
                                    margin = unit(c(t=0.15,r=0,b=0.15,l=0), "cm")),
        strip.background = element_blank()) +
  facet_wrap(~ factor("dummy"), 
             labeller = labeller(`factor("dummy")` = "(C) Third plot")) +
  coord_cartesian(ylim = range(mtcars$wt)) # 同步y轴范围

# 合并图形：启用垂直对齐，保证轴完全对齐
combined_plot <- p1 + p2 + 
  plot_layout(ncol = 2, align = "v")

关键调整说明 #

• 标签对齐：把p2的标题换成单个分面的strip文本，让(C)和(A)(B)处于完全相同的布局位置，实现精准对齐
• y轴对齐：
  1. 用coord_cartesian强制两个图使用相同的y轴范围，避免数据范围差异导致的轴偏移
  2. 统一两个图的aspect.ratio，确保y轴的视觉长度一致
  3. 合并时添加align="v"参数，让两个图的垂直轴完全对齐

内容的提问来源于stack exchange，提问作者Giuseppe Petri