如何用ggplot2为facet_wrap分面添加分组子标题并同行展示

想要用ggplot2的facet_wrap将多个小提琴图整合，数据包含color和group两个分组变量，每个color类别下对应不同的group类别。当前用facet_wrap(color ~ group)实现了分面，但每个分面都会重复显示color和group名称，需求是：

• 相同color的分面处于同一行
• 每个color仅在对应行的顶部显示一次，无需每个分面重复显示

现有示例数据和绘图代码如下：

示例数据 #

score<-c(1:10,1:10,1:10,1:10,1:10,1:10,1:10,1:10,1:10,1:10,1:10,1:10)
  
color<-c("white","white","white","white","white","white","white","white","white","white","white","white","white","white","white","white","white","white","white","white","white","white","white","white","white","white","white","white","white","white","black","black","black","black","black","black","black","black","black","black","black","black","black","black","black","black","black","black","black","black","black","black","black","black","black","black","black","black","black","black","black","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","grey","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown","brown")

group<-c("A","A","A","A","A","A","A","A","A","A","A","A","A","A","A","B","B","B","B","B","B","B","B","B","B","B","B","B","B","B","C","C","C","C","C","C","C","C","C","C","C","C","C","C","C","C","D","D","D","D","D","D","D","D","D","E","E","E","E","E","F","F","F","F","F","F","F","F","F","F","F","F","F","F","F","G","G","G","G","G","G","G","G","G","G","G","G","G","G","G","H","H","H","H","H","H","H","H","H","H","H","H","H","H","H","I","I","I","I","I","I","I","I","I","I","I","I","I","I","I")

animal<-c("dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","dog","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","cat","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny","bunny")

df<-tibble(animal,score,group,color)

当前绘图代码 #

ggplot(df,aes(x=animal,y=score,fill=animal)) + 
geom_violin()+  
facet_wrap(color ~ group)+
theme_clean()

可以通过两种方式实现需求：

方法1：使用facet_grid（ggplot2原生） #

facet_grid支持按行和列分别指定分组变量，将color设为行变量，group设为列变量，同一color的分面会自动处于同一行，且color仅在每行的左侧显示一次。如果希望color标题显示在顶部，可通过主题参数调整：

ggplot(df,aes(x=animal,y=score,fill=animal)) + 
  geom_violin()+  
  facet_grid(rows = vars(color), cols = vars(group), scales = "free_x") +
  theme_clean() +
  theme(
    strip.placement = "outside",
    strip.text.y = element_text(angle = 0)
  )

方法2：使用ggh4x包的嵌套分面（更贴合需求） #

ggh4x的facet_nested支持嵌套分面，可将color作为上层分面，group作为下层分面，实现同一color的分面在同一行，且color标题仅横跨该行顶部显示一次：

首先安装并加载ggh4x包：

install.packages("ggh4x")
library(ggh4x)

然后绘图：

ggplot(df,aes(x=animal,y=score,fill=animal)) + 
  geom_violin()+  
  facet_nested(color ~ group, scales = "free_x") +
  theme_clean()

这种方式下，同一color的所有group分面会排列在同一行，color标题会覆盖该行所有分面的顶部，完全匹配需求。

内容的提问来源于stack exchange，提问作者alex