如何在ggplot2中显式指定分组以实现position_dodge偏移

需求 #

我想要生成的图表效果是：按gender分组对数据点和线条进行横向偏移，同性别下的不同subject线条保持对齐，不会额外偏移。

现有问题：position_dodge行为不符合预期 #

当使用position_dodge()时，线条层不仅会按gender偏移，还会按subject分组偏移，导致同性别下的线条分散开：

library(ggplot2)
dat <- structure(list(subject = structure(c(1L, 1L, 1L, 1L, 2L, 2L, 
2L, 2L, 3L, 3L, 3L, 3L, 4L, 4L, 4L, 4L, 12L, 12L, 12L, 12L, 13L, 
13L, 13L, 13L, 14L, 14L, 14L, 14L, 15L, 15L, 15L, 15L), levels = c("1", 
"2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12", "13", 
"14", "15", "16", "17", "18", "19", "20", "21", "22", "23", "24", 
"25", "26", "27"), class = "factor"), gender = structure(c(1L, 
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 
2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L), levels = c("F", 
"M"), class = "factor"), age = c(8, 10, 12, 14, 8, 10, 12, 14, 
8, 10, 12, 14, 8, 10, 12, 14, 8, 10, 12, 14, 8, 10, 12, 14, 8, 
10, 12, 14, 8, 10, 12, 14), y = c(21, 20, 21.5, 23, 21, 21.5, 
24, 25.5, 20.5, 24, 24.5, 26, 23.5, 24.5, 25, 26.5, 26, 25, 29, 
31, 21.5, 22.5, 23, 26.5, 23, 22.5, 24, 27.5, 25.5, 27.5, 26.5, 
27)), row.names = c(NA, -32L), class = "data.frame")

ggplot(dat, aes(x = age, y = y, color = gender)) +
  geom_point(position = position_dodge(width = 0.6)) +
  geom_line(aes(group = subject), position = position_dodge(width = 0.6))

临时解决方案：position_nudge，但扩展性差 #

用position_nudge可以实现需求，但当偏移因子有多个水平时，需要手动设置每个水平的偏移量，操作繁琐：

ggplot(dat, aes(x = age, y = y, color = gender)) +
  geom_point(position = position_nudge(x = ifelse(dat$gender == "M", 0.1, -0.1), y = 0)) +
  geom_line(aes(group = subject),
            position = position_nudge(x = ifelse(dat$gender == "M", 0.1, -0.1), y = 0))

期望的解决方案 #

有没有办法可以显式指定用于偏移的分组变量？比如类似以下的语法：

ggplot(dat, aes(x = age, y = y, color = gender)) +
  geom_point(position = position_dodge(width = 0.6, group = gender)) +
  geom_line(aes(group = subject),
            position = position_dodge(width = 0.6, group = gender))

感谢帮助。

内容的提问来源于Stack Exchange，提问作者retodomax