# Histogram + Density Plot Combo in R

September 27, 2012
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(This article was first published on 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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