 StanLe -4 years ago 292
R Question

# Overlay normal curve to histogram in R

I have managed to find online how to overlay a normal curve to a histogram in R, but I would like to retain the normal "frequency" y-axis of a histogram. See two code segments below, and notice how in the second, the y-axis is replaced with "density". How can I keep that y-axis as "frequency", as it is in the first plot.

AS A BONUS: I'd like to mark the SD regions (up to 3 SD) on the density curve as well. How can I do this? I tried

`abline`
, but the line extends to the top of the graph and looks ugly.

``````g = d\$mydata
hist(g)
`````` ``````g = d\$mydata
m<-mean(g)
std<-sqrt(var(g))
hist(g, density=20, breaks=20, prob=TRUE,
xlab="x-variable", ylim=c(0, 2),
main="normal curve over histogram")
curve(dnorm(x, mean=m, sd=std),
col="darkblue", lwd=2, add=TRUE, yaxt="n")
`````` See how in the image above, the y-axis is "density". I'd like to get that to be "frequency". StanLe
Answer Source

Here's a nice easy way I found:

``````h <- hist(g, breaks = 10, density = 10,
col = "lightgray", xlab = "Accuracy", main = "Overall")
xfit <- seq(min(g), max(g), length = 40)
yfit <- dnorm(xfit, mean = mean(g), sd = sd(g))
yfit <- yfit * diff(h\$mids[1:2]) * length(g)

lines(xfit, yfit, col = "black", lwd = 2)
``````
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