如何在ggplot柱状图的指定条形上添加单个百分比标签

问题背景 #

绘制分组柱状图时，需仅为每组的第二个条形（number_of_courses为Courses With Issues的类别）添加百分比标签，而非所有条形都显示标签。现有数据如下：

structure(list(source = c("College Provided", "College Provided", 
"Corporate", "Corporate", "Course Production", "Course Production"
), divided_column = c(36.8421052631579, 36.8421052631579, 50, 
50, 75.609756097561, 75.609756097561), number_of_courses = structure(c(1L, 
2L, 1L, 2L, 1L, 2L), levels = c("Total Courses", "Courses With Issues"
), class = "factor"), value = c(19L, 7L, 6L, 3L, 41L, 31L)), row.names = c(NA, 
-6L), class = c("tbl_df", "tbl", "data.frame"))

解决方案 #

可以通过两种方式实现需求：

方法一：用条件判断控制标签显示 #

在geom_text的label参数中，通过ifelse函数仅为目标类别生成标签，其余情况返回空字符串：

ggplot(total_courses_issues_long,  
       aes(x = fct_infreq(source), y = value, fill = number_of_courses)) + 
  geom_bar(stat = "identity", position = "dodge", width = 0.6) + 
  geom_text(aes(label = ifelse(number_of_courses == "Courses With Issues", 
                               paste0(round(divided_column, 2), "%"), "")), 
            position = position_dodge(width = 0.9), 
            vjust = -0.5, size = 3)

方法二：筛选数据集传入geom_text #

直接给geom_text传入仅包含目标类别的筛选后数据集，从源头上限制标签的生成范围：

ggplot(total_courses_issues_long,  
       aes(x = fct_infreq(source), y = value, fill = number_of_courses)) + 
  geom_bar(stat = "identity", position = "dodge", width = 0.6) + 
  geom_text(data = subset(total_courses_issues_long, number_of_courses == "Courses With Issues"),
            aes(label = paste0(round(divided_column, 2), "%")), 
            position = position_dodge(width = 0.9), 
            vjust = -0.5, size = 3)

原理说明 #

• 方法一利用条件逻辑，仅在匹配目标条形时输出有效标签，空字符串不会在图中显示任何内容。
• 方法二通过subset函数过滤数据，只让geom_text处理需要添加标签的行，效率更高且逻辑更清晰。

内容的提问来源于stack exchange，提问作者user15362056