如何在ggplot2中绘制x轴下方带箭头的指示线？

问题分析 #

核心问题是：ggplot2默认会根据数据范围自动设置坐标轴范围，你的数据y值均为正数，因此y轴默认不包含负区间，导致你设置的y=-1、y=-1.5的箭头和文本被裁剪，无法显示。另外，你的数据处理代码存在语法错误：df %> 需要改为 df %>%，否则无法正常运行。

解决方案 #

方案1：扩展y轴范围（箭头在绘图区域内） #

通过手动设置y轴范围，包含负区间，让箭头和标注显示在主绘图区域内：

library(ggplot2)
library(grid)
library(tidyr)

# 数据定义与处理（修正管道符错误）
st <- c("a1", "a2", "a3", "a4", "a5", "a6", "a7", "a8", "a9", "a10")
y1 <- c(5, 1, 4, 6, 2, 3, 7, 8, 2, 8)
y2 <- c(3, 3, 3, 3, 4, 4, 5, 5, 7, 7)
df <- data.frame(st = st, y1 = y1, y2 = y2)

df_long <- df %>%
  pivot_longer(
    cols = c("y1", "y2"),
    names_to = "variable",
    values_to = "value"
  )

ggplot(data = df_long, aes(x = st, y = value, color = variable, group = variable)) +
  geom_line(size = 1.2) +
  labs(
    title = "Line Plot with Arrow Beneath X-Axis",
    x = "Identifier",
    y = "Value",
    color = "Variable"
  ) +
  theme_minimal() +
  # 扩展y轴范围，包含负方向元素
  scale_y_continuous(limits = c(-2, max(df_long$value) + 0.5)) +
  geom_segment(
    x = 1, y = -1, xend = 10, yend = -1,
    arrow = arrow(type = "closed", length = unit(0.2, "inches")),
    color = "blue",
    size = 1.2
  ) +
  annotate("text", x = 3, y = -1.5, label = "Inner", color = "blue", size = 4) +
  annotate("text", x = 8, y = -1.5, label = "Outer", color = "red", size = 4)

方案2：将元素放在绘图区域外（更美观，不压缩主图空间） #

如果希望箭头和标注真正位于x轴下方的空白区域（而非主绘图区域内），可以通过关闭绘图区域裁剪、调整边距，结合grid包的grob对象实现：

library(ggplot2)
library(grid)
library(tidyr)

# 数据定义与处理（修正管道符错误）
st <- c("a1", "a2", "a3", "a4", "a5", "a6", "a7", "a8", "a9", "a10")
y1 <- c(5, 1, 4, 6, 2, 3, 7, 8, 2, 8)
y2 <- c(3, 3, 3, 3, 4, 4, 5, 5, 7, 7)
df <- data.frame(st = st, y1 = y1, y2 = y2)

df_long <- df %>%
  pivot_longer(
    cols = c("y1", "y2"),
    names_to = "variable",
    values_to = "value"
  )

# 创建箭头和文本的grob对象（使用npc坐标适配绘图区域）
arrow_grob <- segmentsGrob(
  x0 = unit(0.1, "npc"), x1 = unit(0.9, "npc"),
  y0 = unit(-0.1, "npc"), y1 = unit(-0.1, "npc"),
  arrow = arrow(type = "closed", length = unit(0.2, "inches")),
  gp = gpar(col = "blue", lwd = 1.2)
)

inner_text <- textGrob(
  label = "Inner", x = unit(0.3, "npc"), y = unit(-0.2, "npc"),
  gp = gpar(col = "blue", fontsize = 12)
)

outer_text <- textGrob(
  label = "Outer", x = unit(0.8, "npc"), y = unit(-0.2, "npc"),
  gp = gpar(col = "red", fontsize = 12)
)

ggplot(data = df_long, aes(x = st, y = value, color = variable, group = variable)) +
  geom_line(size = 1.2) +
  labs(
    title = "Line Plot with Arrow Beneath X-Axis",
    x = "Identifier",
    y = "Value",
    color = "Variable"
  ) +
  theme_minimal() +
  # 关闭绘图区域裁剪，允许显示区域外元素
  coord_cartesian(clip = "off") +
  # 增加底部边距，为箭头和文本预留空间
  theme(plot.margin = margin(t = 10, r = 10, b = 40, l = 10, unit = "pt")) +
  # 添加自定义grob元素
  annotation_custom(grob = arrow_grob) +
  annotation_custom(grob = inner_text) +
  annotation_custom(grob = outer_text)

关键说明 #

• 方案1简单直接，但会让主绘图区域下方出现空白，压缩数据可视化的空间；
• 方案2通过coord_cartesian(clip = "off")关闭裁剪，结合npc归一化坐标定位元素，既实现了x轴下方的标注需求，又不影响主图布局。

内容的提问来源于stack exchange，提问作者Umar