ggplot: bin limits and ticks are misaligned

Question

Look at the data and graph in this example:

df <- data.frame(x = round(rnorm(10000, mean=100, sd=15)))

df$x <- ifelse(df$x <  50,  50, df$x)
df$x <- ifelse(df$x > 150, 150, df$x)

library(ggplot2)

ggplot(df) +
  aes(x = x) +
  geom_histogram(aes(y = ..density..),
                 binwidth = 10,
                 fill="#69b3a2",
                 color="#e9ecef", alpha=0.9) +
  stat_function(fun = dnorm, args = list(mean = mean(df$x), 
                                         sd = sd(df$x)))

The resulting graph is:

Note that the histogram goes outside the data bounds. The data is explicitly set to be limited to the 50 to 150 range, but the histogram seems to represent data from 45 to 155. In other words, the binning seems to be wrong. Also note that the normal curves stops at the correct limits.

Is there a way to change the binning so that the bins go in the correct boundaries?

comment: I have found work-arounds such as this ggplot axis ticks fall at center of bin value rather than at the bin limits but if I understand correctly, the idea here is to move the data by half the bin width, but that would be wrong at the other side in this case. It would also ruin the normal curve)

The data and the representation are correct. The **width** of each bar (percefectly centered in 50 and 150) extends beyond the center so you can appreciate it. You provided a binwidth of 10, wich means the bar will extend 5 to the left and 5 to the right. Consider playing with the binwidth. — PavoDive, Jan 01 '23 at 17:40
You may want to check this, too: https://groups.google.com/g/ggplot2/c/yTEm-OfsX18?pli=1 — PavoDive, Jan 01 '23 at 17:43

zephryl · Accepted Answer · 2023-01-01T18:05:48.423

The default binning isn’t “wrong” per se, but you can control bin alignment by setting the boundary for one of your bins, using the boundary arg for geom_histogram():

library(ggplot2)

ggplot(df) +
  aes(x = x) +
  geom_histogram(aes(y = ..density..),
                 binwidth = 10,
                 fill="#69b3a2",
                 color="#e9ecef", 
                 boundary = 50,
                 alpha=0.9) +
  stat_function(fun = dnorm, args = list(mean = mean(df$x), 
                                         sd = sd(df$x)))

You could alternatively set center = 55 for the same result.

ggplot: bin limits and ticks are misaligned

1 Answers1