如何在R ggplot柱状图图例下方添加分组总计数标注

使用如下数据集与ggplot2代码绘制性别分组的长度频率柱状图，需要在图例区域添加分组总计数标注，两个分组合计总样本量n=275。

示例数据集 #

structure(list(Geslacht = c("man", "man", "man", "man", "man", 
"man", "man", "vrouw", "vrouw", "vrouw", "vrouw", "vrouw", "vrouw", 
"vrouw", "vrouw"), Lengteklasse = c(6, 7, 8, 9, 10, 11, 12, 6, 
7, 8, 9, 10, 11, 12, 13), Freq = c(1L, 9L, 19L, 21L, 35L, 22L, 
5L, 2L, 13L, 19L, 37L, 45L, 40L, 6L, 1L)), class = c("grouped_df", 
"tbl_df", "tbl", "data.frame"), row.names = c(NA, -15L), groups = structure(list(
    Geslacht = c("man", "vrouw"), .rows = structure(list(1:7, 
        8:15), ptype = integer(0), class = c("vctrs_list_of", 
    "vctrs_vctr", "list"))), class = c("tbl_df", "tbl", "data.frame"
), row.names = c(NA, -2L), .drop = TRUE))

初始绘图代码 #

ggplot(Freq_lengteklasse, aes(Lengteklasse, Freq, fill = Geslacht))+
  geom_col()+
  scale_x_continuous(breaks = seq(6,13, by = 1))+
  scale_y_continuous(breaks = seq(0,80, by = 10))+
  labs(x = "Lengteklasse (cm)", y = "Frequentie")+
  scale_fill_manual(values=c('lightgray','black'))+
  theme_classic()

实现方案 #

不需要依赖第三方扩展包，直接调整ggplot的标度和主题参数就能实现，操作如下：

1. 提前计算需要标注的统计量

library(dplyr)
# 计算分性别频数、总频数
n_man <- sum(Freq_lengteklasse$Freq[Freq_lengteklasse$Geslacht == "man"])
n_vrouw <- sum(Freq_lengteklasse$Freq[Freq_lengteklasse$Geslacht == "vrouw"])
total_n <- n_man + n_vrouw # 计算得总样本量为275

2. 利用换行符\n把标注内容嵌入图例的标题或标签中，配合主题参数调整行高避免文字重叠：

• 如果要把总n=275放在图例标题下方，直接修改labs()里的fill标题即可：

ggplot(Freq_lengteklasse, aes(Lengteklasse, Freq, fill = Geslacht))+
  geom_col()+
  scale_x_continuous(breaks = seq(6,13, by = 1))+
  scale_y_continuous(breaks = seq(0,80, by = 10))+
  labs(
    x = "Lengteklasse (cm)", 
    y = "Frequentie",
    fill = paste0("Geslacht\nn = ", total_n)
  )+
  scale_fill_manual(values=c('lightgray','black'))+
  theme_classic()+
  theme(legend.title = element_text(lineheight = 1.2))

• 如果需要在每个分组名下方标注对应组的样本量，修改scale_fill_manual()的labels参数即可：

ggplot(Freq_lengteklasse, aes(Lengteklasse, Freq, fill = Geslacht))+
  geom_col()+
  scale_x_continuous(breaks = seq(6,13, by = 1))+
  scale_y_continuous(breaks = seq(0,80, by = 10))+
  labs(x = "Lengteklasse (cm)", y = "Frequentie")+
  scale_fill_manual(
    values=c('lightgray','black'),
    labels = c(
      paste0("man\nn = ", n_man),
      paste0("vrouw\nn = ", n_vrouw)
    )
  )+
  theme_classic()+
  theme(legend.text = element_text(lineheight = 1.2))

• 如果需要把总样本量放在所有图例条目的最下方，只需要把总n的文本拼接在最后一个分组的标签后，多加一个换行符留出间距即可。

这种方式的标注位置会跟随图例自动调整，不会出现错位问题。

内容的提问来源于stack exchange，提问作者Pepijn95