ggplot2同一坐标轴设置不同长度刻度线的实现方法咨询

方案1：使用ggh4x包实现（最简便） #

ggh4x包扩展了ggplot2的坐标轴功能，支持直接给刻度长度传向量，适配示例数据的代码如下：

library("ggplot2")
library("ggh4x")

set.seed(5)
df <- data.frame(x = rnorm(500, mean = 12.5, sd = 3))
breaks <- seq(2.5, 25, .5)
# 给每一个断点分配对应的刻度长度，主刻度（2.5倍数）长度0.2cm，次刻度0.1cm
tick_len <- ifelse(breaks %% 2.5 == 0, unit(0.2, "cm"), unit(0.1, "cm"))

ggplot(df, aes(x = x)) +
  geom_histogram(breaks = breaks, fill = "gray66", color = "black") +
  scale_x_continuous(
    breaks = breaks,
    # 仅主刻度显示标签，次刻度标签为空
    labels = ifelse(breaks %% 2.5 == 0, breaks, "")
  ) +
  guides(
    x = guide_axis(
      ticks_length = tick_len,
      # 对应设置刻度线粗细，和base R效果对齐
      ticks_linewidth = ifelse(breaks %% 2.5 == 0, 2, 1)
    )
  ) +
  theme_bw()

方案2：原生ggplot2实现（无需额外依赖） #

如果不想安装第三方包，可以通过自定义坐标轴grobs的方式修改，代码如下：

library("ggplot2")
library("grid")

set.seed(5)
df <- data.frame(x = rnorm(500, mean = 12.5, sd = 3))
breaks <- seq(2.5, 25, .5)

p <- ggplot(df, aes(x = x)) +
  geom_histogram(breaks = breaks, fill = "gray66", color = "black") +
  scale_x_continuous(
    breaks = breaks,
    labels = ifelse(breaks %% 2.5 == 0, breaks, "")
  ) +
  theme_bw() +
  theme(axis.ticks.length.x = unit(0.2, "cm"))

# 提取坐标轴grob修改刻度长度
g <- ggplotGrob(p)
axis_idx <- which(g$layout$name == "axis-b")
ticks <- g$grobs[[axis_idx]]$children[[2]]
# 修改刻度y坐标，次刻度长度减半
ticks$y <- unit.c(
  unit(rep(0, sum(breaks %% 2.5 == 0)), "npc"),
  unit(rep(0.5, sum(breaks %% 2.5 != 0)), "npc"),
  rep(unit(1, "npc"), length(breaks))
)
ticks$x <- rep(ticks$x, each = 2)
ticks$id.lengths <- rep(2, length(breaks))
# 修改刻度粗细
ticks$gp$lwd <- rep(ifelse(breaks %% 2.5 == 0, 2, 1), each = 2)
g$grobs[[axis_idx]]$children[[2]] <- ticks

# 绘制最终图形
grid.draw(g)

两种方案都可以实现和base R示例完全一致的长短刻度效果。核心原因是原生ggplot2的theme参数全局控制刻度长度，不支持向量化赋值，所以要么用扩展包的增强坐标轴功能，要么手动修改底层grid图形对象的属性。

内容的提问来源于stack exchange，提问作者Jun Jiang