将图例添加到 R 中的密度图

Question

我在 R 中使用以下代码在一张图上绘制两条密度曲线；

mydata1<-read.csv(file="myfile1.csv",head=TRUE,sep=",")
mydata2<-read.csv(file="myfile2.csv",head=TRUE,sep=",")

pdf("comparison.pdf")

plot.multi.dens <- function(s)   
{
    junk.x = NULL
    junk.y = NULL
    for(i in 1:length(s)) {
        junk.x = c(junk.x, density(s[[i]])$x)
        junk.y = c(junk.y, density(s[[i]])$y)
    }
    xr <- range(junk.x)
    yr <- range(junk.y)
    plot(density(s[[1]]), xlim = xr, ylim = yr, xlab="Usage",main = "comparison")
    for(i in 1:length(s)) {
        lines(density(s[[i]]), xlim = xr, ylim = yr, col = i)
    }
}

plot.multi.dens( list(mydata2$usage,mydata1$usage))    
dev.off()

现在的问题是正在生成的图表显示两条线，但该图表不包含哪条线是哪条线的信息。例如，在输出中，应该显示红线是“a”，黑线是“b”。我是 R 的新手，这就是为什么我遇到了一些困难。任何帮助将不胜感激！

Answer 1

来自 quickR 网站的回答

# Compare MPG distributions for cars with 
# 4,6, or 8 cylinders
library(sm)
attach(mtcars)

# create value labels 
cyl.f <- factor(cyl, levels= c(4,6,8),
    labels = c("4 cylinder", "6 cylinder", "8 cylinder")) 

# plot densities 
sm.density.compare(mpg, cyl, xlab="Miles Per Gallon")
title(main="MPG Distribution by Car Cylinders")

# add legend via mouse click
colfill<-c(2:(2+length(levels(cyl.f)))) 
legend(locator(1), levels(cyl.f), fill=colfill)