**Mollie's Research Blog**, and kindly contributed to R-bloggers)

Plotting a histogram using *hist* from the *graphics* package is pretty straightforward, but what if you want to view the density plot on top of the histogram? This combination of graphics can help us compare the distributions of groups.

Let’s use some of the data included with R in the package *datasets*. It will help to have two things to compare, so we’ll use the *beaver* data sets, *beaver1* and *beaver2:* the body temperatures of two beavers, taken at 10 minute intervals.

First we want to plot the histogram of one beaver:

hist(beaver1$temp, # histogram

col="peachpuff", # column color

border="black",

prob = TRUE, # show densities instead of frequencies

xlab = "temp",

main = "Beaver #1")

Next, we want to add in the density line, using *lines*:

hist(beaver1$temp, # histogram

col="peachpuff", # column color

border="black",

prob = TRUE, # show densities instead of frequencies

xlab = "temp",

main = "Beaver #1")

lines(density(beaver1$temp), # density plot

lwd = 2, # thickness of line

col = "chocolate3")

Now let’s show the plots for both beavers on the same image. We’ll make a histogram and density plot for Beaver #2, wrap the graphs in a *layout *and *png*, and change the x-axis to be the same, using *xlim*.

Here’s the final code, also available on gist:

png("beaverhist.png")

layout(matrix(c(1:2), 2, 1,

byrow = TRUE))

hist(beaver1$temp, # histogram

col = "peachpuff", # column color

border = "black",

prob = TRUE, # show densities instead of frequencies

xlim = c(36,38.5),

xlab = "temp",

main = "Beaver #1")

lines(density(beaver1$temp), # density plot

lwd = 2, # thickness of line

col = "chocolate3")

hist(beaver2$temp, # histogram

col = "peachpuff", # column color

border = "black",

prob = TRUE, # show densities instead of frequencies

xlim = c(36,38.5),

xlab = "temp",

main = "Beaver #2")

lines(density(beaver2$temp), # density plot

lwd = 2, # thickness of line

col = "chocolate3")

dev.off()

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