R 编程中的分层箱线图

一种 boxplot是一组数值数据通过其四分位数的图形表示。箱线图是非参数的，它们显示统计总体样本的变化，而不对基础统计分布进行任何假设。箱线图中箱体不同部分之间的间距表示数据的分散程度和偏度，并显示异常值。箱线图可以垂直或水平绘制。 Boxplot 的名字来自中间的盒子。分层箱线图用于检查分类变量和数字变量之间、由第三个分类变量定义的层或组之间的关系。在比较分类变量时，分层箱线图很有用。

在 R 中的实现

在 R 编程中，可以使用R Graphics Package的boxplot()函数形成分层箱线图。

Syntax: 

boxplot(formula, data = NULL, …, subset, na.action = NULL, xlab = mklab(y_var = horizontal),

             ylab = mklab(y_var =!horizontal), add = FALSE, ann = !add, horizontal = FALSE, drop = FALSE,

             sep = “.”, lex.order = FALSE)

boxplot(x, …, range = 1.5, width = NULL, varwidth = FALSE, notch = FALSE, outline = TRUE, names, plot = TRUE,

             border = par(“fg”), col = NULL, log = “”, pars = list(boxwex = 0.8, staplewex = 0.5, outwex = 0.5),

             ann = !add, horizontal = FALSE, add = FALSE, at = NULL) 

Parameter	Description
formula	a formula.
data	a data.frame/list from which the variables in the formula should be taken.
subset	an optional vector specifying a subset of observations to be used for plotting.
na.action	a function which indicates what should happen when the data contain NAs.
xlab,ylab	x- and y-axis annotation. Can be suppressed by ann=FALSE.
add	logical, if true add boxplot to the current plot.
ann	logical indicating if axes should be annotated (by xlab and ylab).
horizontal	logical indicating if the boxplots should be horizontal; default FALSE means vertical boxes.
x	for specifying data from which the boxplots are to be produced.  Either a numeric vector or a single list containing such vectors.
range	this determines how far the plot whiskers extend out from the box.
width	a vector giving the relative widths of the boxes making up the plot.
varwidth	if varwidth is TRUE, the boxes are drawn with widths proportional to  the square-roots of the number of observations in the groups.
notch	if the notch is TRUE, a notch is drawn in each side of the boxes.
outline	if the outline is not true, the outliers are not drawn.
names	group labels that will be printed under each boxplot.
boxwex	a scale factor to be applied to all boxes.
staplewex	staple line width expansion, proportional to box width.
outwex	outlier line width expansion, proportional to box width.
plot	if TRUE (the default) then a boxplot is produced. Else the summaries  which the boxplots are based on are returned.
border	an optional vector of colors for the outlines of the boxplots.
cols	if col is non-null it is assumed to contain colors to be used to color  the bodies of the box plots.
logs	character indicating if x or y or both coordinates should be plotted in log scale.
pars	a list of (potentially many) more graphical parameters.
at	numeric vector giving the locations where the boxplots should be drawn,  particularly when add = TRUE.
…	for the formula method, named arguments to be passed to the default method.

示例 1：

要绘制分层箱线图，请使用 R 中数据集库的 mtcars 数据集。 mtcars 数据集包含来自 Motor Trend Car Road Tests 的数据。在这里，让我们将不同汽车的里程数（在这种情况下为英里/加仑）与它们所拥有的档位数量进行绘制。

R

# Import the required library
library(datasets)
  
# Import the dataframe
cars <- data.frame(mtcars)
  
# Using boxplot()
boxplot(mpg~gear, data = mtcars,
        main = "Different boxplots for number of gears.",
        xlab = "No.of gears",
        ylab = "Mileage",
        col = "orange",
        border = "brown"
)

示例 2：

我们在这里使用的数据集是 LungCapData 数据集，其中包含不同年龄组吸烟者和非吸烟者的肺活量数据。数据集的结构有 6 个变量，每个变量表示肺活量、年龄、身高、吸烟（吸烟者为“是”，非吸烟者为“否”）、性别（男性/女性）和剖腹产（是/否）一个人的。我们将年龄分组，然后尝试绘制具有年龄层的吸烟者与非吸烟者肺活量的分层箱线图。请在此处下载 CSV 文件。

电阻

# Load the dataset
LungCapData <- read.csv("LungCapData.csv", header = T)
LungCapData <- data.frame(LungCapData)
attach(LungCapData)
  
# Catgorise Age into groups
AgeGroups <- cut(LungCapData$Age,
                 breaks = c(0, 13, 15, 17, 25),
                 labels = c("<13", "14/15", "16/17", ">=18"))
head(LungCapData)
  
# BoxPlot 1
boxplot(LungCapData$LungCap~LungCapData$Smoke,
        ylab = "Capacity", 
        main = "Lung Capacity of Smokers Vs Non-Smokers",
        las = 1)
  
# BoxPlot 2
boxplot(LungCapData$LungCap[LungCapData$Age>=18]~LungCapData$Smoke[LungCapData$Age>=18],
        ylab = "Capacity",
        main = "Lung Capacity of Smokers Vs Non-Smokers",
        las = 1)
  
# BoxPlot 3
boxplot(LungCapData$LungCap~LungCapData$Smoke*AgeGroups,
        ylab = "Capacity", xlab = "",
        main = "Lung Capacity of Smokers Vs Non-Smokers",
        col = c(4, 2), las = 2)

输出：

# 箱线图 1

箱线图 1 绘制了吸烟者和非吸烟者的肺活量，其中 no 代表非吸烟者，yes 代表吸烟者。

通过分析上面显示的箱线图，我们可以清楚地表明，与吸烟者相比，非吸烟者的肺活量平均而言较低。

# 箱线图 2

箱线图 2 绘制了年龄大于或等于 18 岁的吸烟者和非吸烟者的肺活量，其中 no 代表非吸烟者，yes 代表吸烟者。

# 箱线图 3

箱线图 3 绘制了数据集中不同年龄组吸烟者和非吸烟者的肺活量，其中蓝色箱线图代表非吸烟者，红色箱线图代表吸烟者。