ggplot2离散轴设置次刻度及连接geom_point分组点的方法求解

无需额外安装第三方扩展包，仅用tidyverse核心功能即可实现：

• 散点连线：将车型转为数值型坐标后统一设置年份偏移量，新增geom_line层时以「车型+指标」为分组键，即可自动连接每个车型下不同年份的同指标散点
• 双层X轴：将X轴设为连续刻度，上层（靠近绘图区，即车型标签上方）刻度对齐散点位置标注年份；通过文本注释在轴外侧添加车型标签，拉长车型对应中心位置的刻度线做视觉分组，调整边距避免标签裁切。

完整可运行代码 #

library(tidyverse)
# 原示例数据构造逻辑保持不变
data <- mtcars%>%
       as_tibble(rownames = "model")%>%
       mutate(jahr = 2019)%>%
       bind_rows(mtcars%>%
          as_tibble(rownames = "model")%>%
          mutate_if(is.numeric, ~.*0.9)%>%
          mutate(jahr = 2020))%>%
       bind_rows(mtcars%>%
          as_tibble(rownames = "model")%>%
          mutate_if(is.numeric, ~.*0.7)%>%
          mutate(jahr = 2021))

# 固定车型展示顺序
model_levels <- c("Datsun 710", "Honda Civic", "Valiant")

# 预处理绘图数据，生成点位坐标
plot_data <- data%>%
  filter(model %in% model_levels)%>%
  gather(variable, wert, -c(model, jahr))%>%
  mutate(
    model_num = as.numeric(factor(model, levels = model_levels)),
    # 年份偏移量和原代码点位完全对齐
    x_pos = case_when(
      jahr == 2019 ~ model_num - 0.2,
      jahr == 2020 ~ model_num,
      jahr == 2021 ~ model_num + 0.2
    )
  )

ggplot(plot_data, aes(x = x_pos, y = wert, col = model))+
  # 折线层放在散点下层，避免遮挡点位
  geom_line(aes(group = interaction(model, variable)), linewidth = 0.8) +
  geom_point(size = 2.5) +
  facet_wrap(~variable, scales = "free_y") +
  # X轴配置：上层刻度标注年份
  scale_x_continuous(
    breaks = c(0.8, 1, 1.2, 1.8, 2, 2.2, 2.8, 3, 3.2),
    labels = rep(c("2019", "2020", "2021"), 3),
    name = NULL,
    expand = expansion(add = 0.5)
  ) +
  # 添加下层车型标签
  geom_text(
    data = distinct(plot_data, model, model_num),
    aes(x = model_num, y = -Inf, label = model),
    vjust = 2.2, hjust = 0.5, color = "black", size = 3.5
  ) +
  theme(
    # 底部预留足够空间放置两层标签
    plot.margin = margin(b = 25, t = 10, l = 10, r = 10),
    # 车型中心位置刻度线拉长，强化分组视觉效果
    axis.ticks.length.x = unit(c(2, 8, 2, 2, 8, 2, 2, 8, 2), "pt"),
    legend.position = "top"
  )

实现效果 #

1. 同车型、同指标的三个年份散点由同色折线连接，数值变化趋势直观
2. X轴为双层结构：紧邻绘图区的标签为年份，位于车型标签的上方，完全符合轴配置需求。

内容的提问来源于stack exchange，提问作者Waschi Waschoi