R语言：如何用cowplot（nrow=2,ncol=2）消除子图间的空白间隙

问题描述 #

使用R语言cowplot包以2行2列布局拼接子图时，上方两个子图（根长图、根表面积图）的x轴标签位置始终留有空白间隙，即使设置axis.text.x = element_blank()也无法消除，相关代码及输出图如下：

原始代码

# Tukeys字母标记
anova <- aov(Root_Length ~Treatment, data =JO_Data)
tukey <- TukeyHSD(anova)
cld <- multcompLetters4(anova, tukey)

JO_Data_Root_Length <- JO_Data %>% dplyr::group_by(Treatment) %>% dplyr::summarise(mean=mean(Root_Length), sd = sd(Root_Length)) %>% arrange(desc(mean))

cld <- as.data.frame.list(cld$Treatment)
JO_Data_Root_Length$cld <- cld$Letters

# 根长图绘制
plot.JO_Data.Root_Length=ggplot(JO_Data_Root_Length,aes(x = Treatment, y = mean, fill=Treatment)) +
  geom_bar(aes(x=Treatment, y=mean), stat="identity", alpha=1) +
  geom_errorbar( aes(x = Treatment, ymin = mean -sd, ymax = mean +sd), width=0.4, colour="black", alpha=0.9, size=1) +
  geom_text(aes(label = cld, y = mean + sd), vjust = -0.5) +
  labs(y = expression(paste("Root Length (cm)")))+
  scale_y_continuous(expand = c(0,0), limits = c(0,12000), breaks = seq(0, 12000, by = 2000)) +
  scale_x_discrete(limits = c("Control", "Control + UAN 50", "Control + UAN 100","Compost", "Compost + UAN 50", "LD", "LD + UAN 50", "SD", "SD + UAN 50"))+
  scale_fill_manual(values = cbbPalette, breaks = c("Control", "Control + UAN 50", "Control + UAN 100","Compost", "Compost + UAN 50", "LD", "LD + UAN 50", "SD", "SD + UAN 50"))+
  theme(axis.line = element_line(colour = "black"),
        panel.grid.major = element_blank(),
        panel.grid.minor = element_blank(),
        panel.border = element_blank(),
        panel.background = element_blank(), 
        axis.text.x = element_blank(),
        axis.text.y = element_text(size = 12),
        axis.title.y = element_text(size = 12))+  
  theme(legend.justification = c("right", "top"))+
  theme(legend.title = element_text(color = "black", size = 16, face = "bold"))+
  theme(legend.title.align = 0.5) +
  theme(legend.background = element_rect(linetype = "solid", colour = "black", size = 1))+
  theme(legend.margin = margin(0.1,0.2,0.2,0.2, unit ="cm"))+
  theme(legend.text = element_text(size=14, colour="black"))+
  theme(axis.title.x=element_blank())

plot.JO_Data.Root_Length

################### 其余三个图代码与根长图结构一致，此处省略 ########

# 子图拼接
library(gridExtra)
library(cowplot)

JO_plot2 <- plot_grid(plot.JO_Data.Root_Length + theme(legend.position="none"),
                      plot.JO_Data.Root_SA + theme(legend.position="none"),
                      plot.JO_Data.Root_diameter + theme(legend.position="none"),
                      plot.JO_Data.Root_Vol + theme(legend.position="none"),
                      align = "h",
                      axis = "b",
                      hjust = 0.1,
                      vjust = 0.1,
                      nrow = 2,
                      ncol =2)

legend <- get_legend(plot.JO_Data.Root_Length)

Root_plot2 <- plot_grid(JO_plot2, legend, rel_widths = c(1.75, 0.5))
Root_plot2

输出图

解决方案 #

核心问题分析 #

axis.text.x = element_blank()仅移除了x轴文本，但x轴刻度、以及plot_grid中axis = "b"参数强制所有子图底部对齐的设置，会继续保留空白空间。

具体修改步骤 #

1. 移除plot_grid中的axis = "b"参数
   该参数会强制所有子图的底部（x轴）对齐，导致上方子图必须预留与下方子图x轴一致的空白。删除后，上方子图可自由压缩底部空间。

2. 修改上方子图的theme配置
   在根长图和根表面积图的theme中添加以下设置，彻底清除x轴相关空白：

   • axis.ticks.x = element_blank()：移除x轴刻度（默认会占据空间）
   • plot.margin = margin(t=10, r=10, b=0, l=10, unit="pt")：将底部边距设为0，消除最后残留的空白

3. （可选）调整对齐方式
   若需要保证子图左侧y轴对齐，可将align参数设置为"v"（垂直对齐）。

修改后的关键代码片段 #

修改后的根长图theme部分

theme(axis.line = element_line(colour = "black"),
      panel.grid.major = element_blank(),
      panel.grid.minor = element_blank(),
      panel.border = element_blank(),
      panel.background = element_blank(), 
      axis.text.x = element_blank(),
      axis.ticks.x = element_blank(),  # 新增：移除x轴刻度
      axis.text.y = element_text(size = 12),
      axis.title.y = element_text(size = 12),
      plot.margin = margin(t=10, r=10, b=0, l=10, unit="pt")  # 新增：缩小底部边距
) +  
# 其余theme设置保持不变

修改后的plot_grid部分

JO_plot2 <- plot_grid(plot.JO_Data.Root_Length + theme(legend.position="none"),
                      plot.JO_Data.Root_SA + theme(legend.position="none"),
                      plot.JO_Data.Root_diameter + theme(legend.position="none"),
                      plot.JO_Data.Root_Vol + theme(legend.position="none"),
                      align = "v",  # 仅垂直对齐左侧y轴
                      hjust = 0.1,
                      vjust = 0.1,
                      nrow = 2,
                      ncol =2)

内容的提问来源于stack exchange，提问作者James