如何使用ggplot2的facet_wrap，实现分组X轴保留专属因子不丢失？

要让gc1组始终显示A、B、C所有因子水平，ps1组仅显示D、E、F且互不干扰，推荐使用ggh4x包的facetted_pos_scales功能，它支持为不同分面单独配置轴刻度。

方法一：使用ggh4x包（推荐） #

这个方法能确保所有gc1分面都显示A、B、C，不管子分组是否有对应数据，ps1分面同理。

library(ggplot2)
library(ggh4x)

# 生成数据
set.seed(1234)
df = data.frame(ps = paste(rep(c("ps1","gc1"),6)),
                cr = rep(LETTERS[1:3],4),
                id = rep(x = month.abb[1:4], each = 3))
df = df[order(df$ps),]
df[df$ps == "ps1","cr"] <- sample(LETTERS[4:6], size = 6, replace = TRUE)
df.all = rbind(df,df,df,df,df,df)
df.all$val = rnorm(n = nrow(df.all))

# 绘图并设置分面X轴刻度
ggplot(df.all, aes(x = cr, y = val)) + 
  geom_violin() +
  geom_point(position = position_jitter(width = 0.1, height = 0.1), alpha= .3) +
  facet_wrap(vars(ps, id), scales = "free_x", ncol = 4) +
  facetted_pos_scales(
    x = list(
      ps == "gc1" ~ scale_x_discrete(limits = c("A", "B", "C")),
      ps == "ps1" ~ scale_x_discrete(limits = c("D", "E", "F"))
    )
  )

方法二：预处理数据（无需额外包） #

如果不想依赖第三方包，可以通过为每个ps分组固定cr的因子水平来实现：

library(ggplot2)
library(dplyr)

# 生成数据（同上）
set.seed(1234)
df = data.frame(ps = paste(rep(c("ps1","gc1"),6)),
                cr = rep(LETTERS[1:3],4),
                id = rep(x = month.abb[1:4], each = 3))
df = df[order(df$ps),]
df[df$ps == "ps1","cr"] <- sample(LETTERS[4:6], size = 6, replace = TRUE)
df.all = rbind(df,df,df,df,df,df)
df.all$val = rnorm(n = nrow(df.all))

# 预处理：为每个ps分组设置固定的cr因子水平
df.all <- df.all %>%
  group_by(ps) %>%
  mutate(
    cr = case_when(
      ps == "gc1" ~ factor(cr, levels = c("A", "B", "C")),
      ps == "ps1" ~ factor(cr, levels = c("D", "E", "F"))
    )
  ) %>%
  ungroup()

# 绘图
ggplot(df.all, aes(x = cr, y = val)) + 
  geom_violin() +
  geom_point(position = position_jitter(width = 0.1, height = 0.1), alpha= .3) +
  facet_wrap(vars(ps, id), scales = "free_x", ncol = 4)

这种方法下，即使某个gc1的子分组缺少某个水平，对应的分面仍会显示该刻度，但不会有图形元素。

内容的提问来源于stack exchange，提问作者M. Beausoleil