How to overlay density histogram with gamma distribution fit in R?

Question

I am new to R and would like to add a fit to a gamma distribution to my histogram. I would like the gamma distribution fit to overlay my histogram.

I am able to calculate the gamma distribution with the dgamma function and also with the fitdist function. However, I am not able to overlay this gamma distribution as a fit onto my histogram.

This is the code I tried:

hist(mydata, breaks = 30, freq = FALSE, col = "grey")

lines(dgamma(mydata, shape = 1))

The code I tried does not overlay the gamma distribution fit onto my histogram. I only get the histogram without the fit.

Answer 1

See if the following example can help in overlaying

1. a fitted line in black
2. a PDF graph in red, dotted

on a histogram.

First, create a dataset.

set.seed(1234)    # Make the example reproducible
mydata <- rgamma(100, shape = 1, rate = 1)

Now fit a gamma distribution to the data.

param <- MASS::fitdistr(mydata, "gamma")

This vector is needed for the fitted line.

x <- seq(min(mydata), max(mydata), length.out = 100)

And plot them all.

hist(mydata, breaks = 30, freq = FALSE, col = "grey", ylim = c(0, 1))
curve(dgamma(x, shape = param$estimate[1], rate = param$estimate[2]), add = TRUE)
lines(sort(mydata), dgamma(sort(mydata), shape = 1),
      col = "red", lty = "dotted")