Using curve() and dnorm() to overlay histogram

Question

I need to overlay a normal distribution on a histogram using curve() and dnorm(). dnorm should not have the parameters of my data, but be a generalized normal distribution with mean = 0 and sd = 5.

Using this site and a lot of tutorials, I would expect the code to be

hist(mtcars$mpg)
curve(dnorm(x, 0, 5), add= TRUE, col="red")

But all I get is a flat line. If I use a stand alone example, leaving out add=TRUE I get the desired density function:

curve(dnorm(x, 0, 5), col="red")

Does anyone have any idea what mistake I am making?

What is `x`? Can you provide a reproducible example (i.e., all the code needed to get to to your problem)? — riccardo-df, Jan 23 '22 at 18:35
freq=FALSE hasn't changed anything. x is part of curve(), so the example I gave is reproducible and the shortest version of the problem. — rintrah, Jan 23 '22 at 19:12

score 0 · Accepted Answer · answered Jan 23 '22 at 20:20

Over the range of the data (10-35), there is very little probability in the Normal distribution with a mean of 0 and a SD of 5 (i.e. the curve starts about at the upper end of the 95% confidence interval of the distribution).

If we add freq= FALSE to the hist() call (as is appropriate if you want to compare a probability distribution to the histogram), we can see a little bit of the red curve at the beginning (you could also multiply by a constant if you want the tail to be more visible). (Distribution shown for a more plausible value of the mean as well [blue line].)

## png("tmp.png"); par(las=1, bty="l")
hist(mtcars$mpg, freq=FALSE)
curve(dnorm(x, 0, 5), add = TRUE, col = "red")
curve(dnorm(x, 20, 5), add = TRUE, col = "blue")
## dev.off()

Graphically, this might be clearer/more noticeable if you shaded the area under the Normal distribution curve

Ah, I made some mistake with the freq=FALSE Thank you a lot! It was really beginning to drive me mad. — rintrah, Jan 23 '22 at 20:29

Using curve() and dnorm() to overlay histogram

1 Answers1