Adding a line on histogram

Hi, and welcome!

Couple of preliminaries: 1) while your example is simple enough not to be necessary, a reproducible example, called a reprex is usually very helpful to get good answers; 2) if applicable, please see the homework policy.

# load "tidyverse" plotting package
# load data frame enhancement package
# make results reproducible
x <- 1000
# create the random distribution and convert to a tibble
distrib <- enframe(rchisq(x,3))
# crease the base plot object
p <- ggplot(distrib, aes(x = value))
# add command to produce a "faded" histogram and select the number of bins
# then overlay density curve (converted to common axis of count)
p + geom_histogram(fill="black", colour="black", alpha = 0.25, binwidth=0.5) + geom_density(aes(y=0.5*..count..), colour="black", adjust=4) 

Created on 2020-01-10 by the reprex package (v0.3.0)