Normal curves on multiple histograms on a same plot - r

Question

My example dataframe:
sample1 <- seq(100,157, length.out = 50)
sample2 <- seq(113, 167, length.out = 50)
sample3 <- seq(95,160, length.out = 50)
sample4 <-seq(88, 110, length.out = 50)
df <- as.data.frame(cbind(sample1, sample2, sample3, sample4))
I have managed to create histograms for these four variables, which share the same y-axis. Now I need an overlay normal curve. Based on previous posts, I've managed a density curve, but this is not what I want. This comes close, but I'd like a smooth line...
This is my current code for plotting:
df <- as.data.table(df)
new.df<-melt(df,id.vars="sample")
names(new.df)=c("sample","type","value")
cdat <- ddply(new.df, "type", summarise, value.mean=mean(value))
ggplot(data = new.df,aes(x=value)) +
geom_histogram(aes(x = value), bins = 15, colour = "black", fill = "gray") +
facet_wrap(~ type) + geom_density(aes(x = value),alpha=.2, fill="#FF6666") +
geom_vline(data=cdat, aes(xintercept=value.mean),
linetype="dashed", size=1, colour="black") +
theme_classic() +
theme(text = element_text(size = 15), element_line(size = 0.5),aspect.ratio = 0.75 )
And I found the following code, which I hoped would do the trick, but this gives me nothing:
stat_function(fun = dnorm, args = list(mean = mean(df$value), sd = sd(df$value)))

Answer 1

Unfortunately, stat_function doesn't play nicely with facets: it overlays the same function on each facet without taking account of the faceting variable.
One of the most common reasons I see for people posting ggplot questions on Stack Overflow is that they get lost while trying to coerce ggplot to do too much of their data manipulation. Functions like geom_smooth and geom_function are useful helpers for common tasks, but if you want to do something that is complex or uncommon, it is best to produce the data you want to plot, then plot it.
In fact, the main author of ggplot2 recommends this approach for a very similar problem to yours in this thread, saying:
I think you are better off generating the data outside of ggplot2 and then plotting it. See https://speakerdeck.com/jennybc/row-oriented-workflows-in-r-with-the-tidyverse to get started.
Hadley Wickham, 26 April 2018
So here's one way of doing that using tidyverse. You create a data frame of the dnorm for each sample and plot these using plain old geom_line.
Note that your histograms are counts, so you either need to change them to density, or multiply the dnorm output by the number of observations * the binwidth, otherwise you will just get an apparently "flat" line on the x axis, since the dnorm values will all be so small in relation to the counts:
library(plyr)
library(dplyr)
library(tidyr)
library(ggplot2)
dfn <- df %>%
pivot_longer(everything()) %>%
ddply("name", function(x) {
xvar <- seq(min(x$value), max(x$value), length.out = 100)
data.frame(value = xvar,
y = 5 * nrow(x) * dnorm(xvar, mean(x$value), sd(x$value)))
})
df %>%
pivot_longer(everything()) %>%
group_by(name) %>%
mutate(mean = mean(value), sd = sd(value)) %>%
ggplot(aes(value)) +
geom_histogram(aes(x = value), binwidth = 5,
colour = "black", fill = "gray") +
facet_wrap(~ name) +
geom_vline(aes(xintercept = mean),
linetype = "dashed", size=1, colour="black") +
geom_line(data = dfn, aes(y = y)) +
theme_classic() +
theme(text = element_text(size = 15), element_line(size = 0.5),
aspect.ratio = 0.75 )
Created on 2020-12-07 by the reprex package (v0.3.0)