How to create a density plot in R

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# -------------------------------------------------
# How to create a density plot in R
# -------------------------------------------------
# load library and data
library(mlbench)
data(mtcars)
dim(mtcars)

par(mfrow=c(1,1))

# Density Plot
d <- density(mtcars$mpg) 
plot(d, main="Density Plot") 

# Filled Density Plot
d <- density(mtcars$mpg)
plot(d, main="Kernel Density of Miles Per Gallon")
polygon(d, col="red", border="blue")

# 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")
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Package 'sm', version 2.2-5.6: type help(sm) for summary information
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