如何在ggplot2极坐标图中延伸刻度线至主网格线实现标注？

问题描述 #

我想在ggplot2的极坐标图中，通过延伸y轴刻度线使其与主网格线相交来添加标注。已经设置了axis.ticks.length.y参数延长刻度线，但刻度线没能和图中的主网格线衔接上。

现有代码：

library(ggplot2)
scn = c(1, 2, 3, 2, 5)
data_test = structure(list(s = c("1.0", "1.1", "1.2", "1.3", "1.4"),
                           
                           scn = scn, 
                           vv = scn), 
                      row.names = c(NA, -5L), class = c("tbl_df", 
                                                        "tbl", "data.frame"))

ggplot(data = data_test,
       mapping = aes(x = s, 
                     y = vv, 
                     fill = s, 
                     color = s)) +
  geom_col(width = 1) +
  coord_radial(expand = FALSE) +
  expand_limits(y = -2) +
  theme(
    panel.grid.major.x = element_blank() ,
    panel.ontop = TRUE,
    panel.background = element_blank(),
    strip.background = element_blank(),
    axis.ticks.x = element_line(linetype = "blank"),
    axis.ticks.length.y = unit(0.5, "cm"),
    axis.text.x = element_blank(),
    axis.text.y = element_text(size = 10, 
                               colour = 'grey30', 
                               face = 'bold'),
    panel.grid.major.y = element_line( 
      linewidth=.1, color="grey10" , 
      linetype = 'dotted') 
  ) + 
  scale_y_continuous(breaks= c(1:5))

当前效果：

期望效果：

解决方案 #

在coord_radial极坐标下，原生的axis.ticks.length.y参数无法直接让刻度线精准对齐主网格线，原因是极坐标的刻度线长度计算逻辑和笛卡尔坐标不同。可以通过手动绘制刻度线的方式实现需求：

修改后的完整代码 #

library(ggplot2)
scn = c(1, 2, 3, 2, 5)
data_test = structure(list(s = c("1.0", "1.1", "1.2", "1.3", "1.4"),
                           scn = scn, 
                           vv = scn), 
                      row.names = c(NA, -5L), class = c("tbl_df", "tbl", "data.frame"))

# 准备刻度线数据：基于y轴breaks值生成刻度位置
tick_data <- data.frame(y = 1:5)

ggplot(data = data_test,
       mapping = aes(x = s, 
                     y = vv, 
                     fill = s, 
                     color = s)) +
  geom_col(width = 1) +
  # 手动绘制径向刻度线，覆盖所有x类别对应的圆周范围
  geom_segment(data = tick_data,
               aes(x = 0.5, xend = 5.5, y = y, yend = y),
               color = "grey30", linewidth = 0.5) +
  coord_radial(expand = FALSE) +
  expand_limits(y = -2) +
  theme(
    panel.grid.major.x = element_blank() ,
    panel.ontop = TRUE,
    panel.background = element_blank(),
    strip.background = element_blank(),
    axis.ticks.x = element_blank(),
    # 关闭原生y轴刻度线避免重复
    axis.ticks.y = element_blank(),
    axis.text.x = element_blank(),
    axis.text.y = element_text(size = 10, 
                               colour = 'grey30', 
                               face = 'bold'),
    panel.grid.major.y = element_line( 
      linewidth=.1, color="grey10" , 
      linetype = 'dotted') 
  ) + 
  scale_y_continuous(breaks= c(1:5))

关键说明 #

• tick_data包含所有y轴刻度的位置，对应主网格线的径向坐标
• geom_segment的x=0.5到x=5.5覆盖了数据中5个x类别（s的取值）的范围，确保刻度线完整环绕圆周
• 关闭原生axis.ticks.y，避免和手动绘制的刻度线重复
• 手动绘制的刻度线会和主网格线完全重合，实现你想要的衔接效果

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