如何在ggplot2边距中实现位置固定的刻度标签，无需hack调整？

需求说明 #

需要在ggplot2的边距区域添加固定位置的刻度极点标签，同类图表切换时标签不会发生位移。原实现方案需要根据X轴分类水平数动态计算标签位置，属于临时hack，存在适配性差的问题。

可行实现方案 #

无需依赖数据坐标计算位置，直接使用grid包的归一化父坐标（NPC）即可实现，坐标取值范围为0-1，相对于绘图面板的边界定位，完全不受数据维度影响。

改进后代码 #

library(ggplot2)
library(grid)
library(tidyverse)

dat = data.frame("Country" = c(rep("USA",50),
                               rep("Canada",50),
                               rep("Japan",50),
                               rep("Poland",50),
                               rep("Germany",50)),
                 "Outcome" = c(rnorm(n = 50, mean = 4.0, sd = 1.0),
                               rnorm(n = 50, mean = 5.0, sd = 0.5),
                               rnorm(n = 50, mean = 3.0, sd = 1.5),
                               rnorm(n = 50, mean = 4.5, sd = 1.0),
                               rnorm(n = 50, mean = 4.5, sd = 1.5))
)

myfunction = function(data, IV, DV, varlevels){
  # 计算均值用于绘图
  as_tibble(data) %>%
    group_by({{IV}}) %>%
    summarise(Outcome = mean({{DV}},na.rm=T)) %>%
    
    ggplot(mapping = aes({{IV}}, Outcome, fill = {{IV}})) +
    geom_col() +
    scale_x_discrete(limits = {{varlevels}}) +
    scale_y_continuous(breaks = c(1:7), limits = c(1,7), oob = scales::squish) +
    theme(legend.position = "none",
          # 调整左边距避免标签被裁切
          plot.margin = unit(c(0.5,0.5,0.5,1), "lines")) +
    coord_cartesian(clip = "off") +
    # 用NPC坐标定位，x=-0.1代表绘图面板左边界往左10%的位置，固定不变
    annotation_custom(
      grob = textGrob(label = "Good", gp = gpar(col = "grey50"), rot = 90,
                      x = unit(-0.1, "npc"), y = unit(6.5, "native")),
      xmin = -Inf, xmax = Inf, ymin = -Inf, ymax = Inf
    ) +
    annotation_custom(
      grob = textGrob(label = "Bad", gp = gpar(col = "grey50"), rot = 90,
                      x = unit(-0.1, "npc"), y = unit(1.5, "native")),
      xmin = -Inf, xmax = Inf, ymin = -Inf, ymax = Inf
    )
}

# 测试不同分类数的图表，标签位置保持一致
myfunction(data = dat, IV = Country, DV = Outcome, varlevels =  c("USA","Canada","Japan","Poland","Germany"))
ggsave("outcome_by_country.png", dpi = 300, units = "in", height= 5.89, width = 6.69)

方案说明 #

• 标签的X坐标使用unit(-0.1, "npc")定义，相对于绘图面板的左边界定位，无论X轴有多少个分类，该位置都固定不变
• Y坐标使用unit(1.5, "native")保持和原方案一致的Y轴数据坐标定位，符合刻度标签和Y轴刻度对应的需求
• 仅需调整plot.margin的左边距参数避免标签被裁切，无需其他额外适配

内容的提问来源于stack exchange，提问作者sachem87