R 使用“时的文本对齐方式”；表达方式；在ggplot2轴文本中生成特殊字符的步骤

我正在使用ggplot2绘制患者BMI的箱线图，但x轴上的文本有问题。我想包括体重指数（kg/m2）的单位，我想在“2”上加上上标。当我这样制作图表时：

require(plyr)
require(ggplot2)

Weights <- data.frame(SubjectID = 1:500,
                      Weight = rnorm(500, 28, 7))
Weights$Cat <- cut(Weights$Weight, breaks = c(0, 18.5, 25, 30, 40, Inf),
                   right = FALSE)
Weights$BMIcat <- revalue(Weights$Cat, 
                           c("[0,18.5)" = "Underweight\n(<18.5 kg/m2)",
                             "[18.5,25)" = "Normal weight\n(18.5 to 24.9\nkg/m2)",
                             "[25,30)" = "Overweight\n(25 to 29.9\nkg/m2)",
                             "[30,40)" = "Obese\n(30 to 39.9\nkg/m2)",
                             "[40,Inf)" = "Severely obese\n(>40 kg/m2)"))

ggplot(Weights, aes(x = BMIcat, y = Weight)) +
      geom_boxplot() +
      xlab("BMI category") + ylab("Weight (kg)")

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现在，我的2是上标的，但其他一切看起来都很糟糕。看起来它突然向左或可能完全对齐，而不是居中，2有时确实远离m，轴文本与图形重叠！有什么建议吗？

您可以使用

Weights$BMIcat
ggplot(Weights, aes(x = Cat, y = Weight)) +
      geom_boxplot() +
      scale_x_discrete(breaks=c("[0,18.5)", "[18.5,25)", "[25,30)",
                                "[30,40)", "[40,Inf)"),
                       labels=c(
                             expression("Underweight\n(<18.5 kg/m"^2*")"),
                             expression("Normal weight\n(18.5 to 24.9\nkg/m"^2*")"),
                             expression("Overweight\n(25 to 29.9\nkg/m"^2*")"),
                             expression("Obese\n(30 to 39.9\nkg/m"^2*")"),
                             expression("Severely obese\n(>40 kg/m"^2*")"))) +
      xlab("BMI category") + ylab("Weight (kg)")

Weights$BMIcat <- revalue(Weights$Cat, 
                           c("[0,18.5)" = "Underweight\n(<18.5 kg/m²)",
                             "[18.5,25)" = "Normal weight\n(18.5 to 24.9\nkg/m²)",
                             "[25,30)" = "Overweight\n(25 to 29.9\nkg/m²)",
                             "[30,40)" = "Obese\n(30 to 39.9\nkg/m²)",
                             "[40,Inf)" = "Severely obese\n(>40 kg/m²)"))
ggplot(Weights, aes(x = BMIcat, y = Weight)) +
      geom_boxplot() +
      xlab("BMI category") + ylab("Weight (kg)")