Density Plots

R has several functions for generating smoothed histograms, or density plots. We will focus on the "density" function.

Basic usage is starightforward:

          plot(density(Glucose1[,1]))
# or fancier:
          plot(density(Glucose1[,1]), col="red", lwd=2, lty=3,
               main="Glucose Test 1 Density")
          abline(h=0)
          rug(Glucose1[,1])

The density function produces a data structure with several components - the first ($x) is the set of points on the x-axis where the density was evaluated; the second ($y) is the height of the density curve at those points.

          d <- density(Glucose1[,1])
          names(d)

The main issue with density estimates is how much smoothing to do. There is a "bandwidth" parameter (bw) in the density function that determines this.

Try a few different values:

          plot(density(Glucose1[,1],bw=.5),col="blue",lwd=2)

If the bandwidth parameter is too small, you get a punk hair style:

          plot(density(Glucose1[,1],bw=100),col="purple",lwd=2)

If the width parameter is too large, you get a nice smooth curve that looks the same no matter what your data were!

It is often useful to combine a density plot and some other plot such as a histogram or dotplot. The "lines" function adds a lineplot to an existing plot:

          hist(Glucose1[,1],freq=F,col="tan")
          lines(density(Glucose1[,1]),lwd=2,col="red")

You can experiment with different line styles:

          hist(Glucose1[,1],freq=F,col="salmon")
          lines(density(Glucose1[,1]),lty=2,lwd=2,col="blue")

Try the digits 0 through 9 for the lty parameter.

Here is a "dotplot" added to a density curve by the "points" function; the original data values are plotted on the x axis below the density cureve:

          plot(density(Glucose1[,1]))
          points(Glucose1[,1],rep(0,53))

The rug() function does something similar, but nicer.

          plot(density(Glucose1[,1]),col="purple",lwd=2,
               main="Glucose test 1 density")
          rug(Glucose1[,1],lwd=2,col="tomato")

Try both at once!

Math 141 Index

Albyn Jones