ggplot元编程函数报错：如何在ggplot中注入字符型列名？

问题背景 #

想要编写一个函数，从数据框中选取指定列并生成ggplot图形，示例代码如下：

main_df1 <- starwars %>% dplyr::select(name, height, mass)
main_df2 <- starwars %>% dplyr::select(name, sex, gender, 
                                       homeworld, species)


plot_gg <- function(main_df,
                    another_df,
                    column_for_clr = c("sex", "gender", "species", 
              "homeworld")){

  #var_clf <- enquo(column_for_clr)
  new_df <-  main_df %>%
    left_join(another_df, by = "name")

  p_p <- new_df %>%
    ggplot(
      aes(x = height,
          y = mass,
          colour = as.symbol(column_for_clr))
      ) +
    xlab("Height") +
    ylab("Mass")

  classes_to_colors_10 <- c(
    "feminine"        = "red",            #  gender
    "masculine"       = "#3cb44b",      #
    "female"          = "navy",              # sex
    "hermaphroditic"  = "darkgreen",    #
    "male"            = "purple4"               #
  )

  if (column_for_clr == "sex") {
    print("colour is sex")
    p_p <- p_p +
      scale_colour_manual(values = classes_to_colors_10) +
      ggtitle("title ")
  }

  ggsave(plot   = p_p,
         device = "png",
         filename = paste0(getwd(), "/test_starwars21.png"),
         units  = "in",
         width  = 6,
         height = 4,
         dpi    = 300)
  return(p_p)
}
plot_gg(main_df1, main_df2, column_for_clr= "sex")

报错信息 #

运行后出现如下错误：

[1] "colour is sex"
Error in `geom_blank()`:
! Problem while computing aesthetics.
ℹ Error occurred in the 1st layer.
Caused by error in `compute_aesthetics()`:
! Aesthetics are not valid data columns.
✖ The following aesthetics are invalid:
✖ `colour = as.symbol(column_for_clr)`
ℹ Did you mistype the name of a data column or forget to add 
 `after_stat()`?
Run `rlang::last_trace()` to see where the error occurred.
Called from: signal_abort(cnd, .file)

尝试过enquo、rlang::!!()、{{ column_for_clr }}等元编程工具，但始终出现各类错误，希望找到在ggplot中实现字符型列名注入的方法。

解决方案 #

方法1：使用.data代词（官方推荐） #

ggplot支持通过.data[[列名字符串]]的方式直接引用字符型列名，这是最安全且直观的方案，无需复杂元编程：

修改ggplot的aes部分：

p_p <- new_df %>%
  ggplot(
    aes(x = height,
        y = mass,
        colour = .data[[column_for_clr]])
  ) +
  xlab("Height") +
  ylab("Mass")

方法2：使用aes_string（兼容旧写法） #

如果习惯传统写法，也可以用aes_string直接传入字符型参数（该函数已软弃用，但仍可正常使用）：

p_p <- new_df %>%
  ggplot(
    aes_string(x = "height",
               y = "mass",
               colour = column_for_clr)
  ) +
  xlab("Height") +
  ylab("Mass")

完整修改后的函数 #

main_df1 <- starwars %>% dplyr::select(name, height, mass)
main_df2 <- starwars %>% dplyr::select(name, sex, gender, 
                                       homeworld, species)


plot_gg <- function(main_df,
                    another_df,
                    column_for_clr = c("sex", "gender", "species", "homeworld")){

  new_df <-  main_df %>%
    left_join(another_df, by = "name")

  # 使用.data代词的写法
  p_p <- new_df %>%
    ggplot(
      aes(x = height,
          y = mass,
          colour = .data[[column_for_clr]])
    ) +
    xlab("Height") +
    ylab("Mass")

  classes_to_colors_10 <- c(
    "feminine"        = "red",
    "masculine"       = "#3cb44b",
    "female"          = "navy",
    "hermaphroditic"  = "darkgreen",
    "male"            = "purple4"
  )

  if (column_for_clr == "sex") {
    print("colour is sex")
    p_p <- p_p +
      scale_colour_manual(values = classes_to_colors_10) +
      ggtitle("Star Wars Height vs Mass by Sex")
  }

  ggsave(plot   = p_p,
         device = "png",
         filename = paste0(getwd(), "/test_starwars21.png"),
         units  = "in",
         width  = 6,
         height = 4,
         dpi    = 300)
  return(p_p)
}

# 测试函数
plot_gg(main_df1, main_df2, column_for_clr= "sex")

错误原因说明 #

你尝试的enquo、!!、{{ }}是针对非字符型裸变量设计的（比如调用函数时传入sex而非"sex"）。如果要兼容字符型输入，.data代词是最直接的方案，避免了字符串与裸表达式转换的复杂操作。

内容的提问来源于stack exchange，提问作者K Y