Find binwidth of Histogram plotted using ggplot2

Question

I have plotted a simple histogram using ggplot without using the binwidth attribute. However, I want to see the value of binwidth of my histogram.

set.seed(1234)
df <- data.frame(
  sex=factor(rep(c("F", "M"), each=200)),
  weight=round(c(rnorm(200, mean=55, sd=5), rnorm(200, mean=65, sd=5)))
  )
head(df)

library(ggplot2)

ggplot(df, aes(x=weight)) + geom_histogram()

How can I view this value?

THanks

Answer 1

Here are two ways.

1. With ggplot_build create an list object. Its 1st member has a data.frame data with xmin and xmax. The differences between these values is the binwidth;
2. With layer_data the process above is more direct. It extracts the data.frame and the rest is the same.

The return value of unique is not a vector of length 1 due to floating-point precision issues. Any value can be used.

set.seed(1234)
df <- data.frame(
  sex=factor(rep(c("F", "M"), each=200)),
  weight=round(c(rnorm(200, mean=55, sd=5), rnorm(200, mean=65, sd=5)))
)

library(ggplot2)

gg <- ggplot(df, aes(x=weight)) + geom_histogram()

gg_build <- ggplot_build(gg)
#> `stat_bin()` using `bins = 30`. Pick better value with `binwidth`.
bin_width <- gg_build$data[[1]]$xmax - gg_build$data[[1]]$xmin
unique(bin_width)
#> [1] 1.344828 1.344828 1.344828

diff(range(df$weight))/30
#> [1] 1.3

gg_data <- layer_data(gg)
#> `stat_bin()` using `bins = 30`. Pick better value with `binwidth`.
unique(gg_data$xmax - gg_data$xmin)
#> [1] 1.344828 1.344828 1.344828

bw <- unique(gg_data$xmax - gg_data$xmin)[1]
bw
#> [1] 1.344828

^{Created on 2022-02-15 by the reprex package (v2.0.1)}