columns are not aligned with the data in ggplot geom_col

Question

plotting some data to a figure with numerical data, one would expect the column border to line up with the grid. However, when plotting this data, you can see that there are some that line up correctly (10, 5), but others don't (2, 1).

Is this a bug or a feature?

Reproducible example

library(tidyverse, scales)

The data

x1 <- c("a", "b", "c", "d", "e")
y1 <- c(1, 10, 2, 1, 5)
xy <- data.frame(x1, y1)

The plot

xy %>% 
  ggplot(aes(x = fct_reorder(x1, desc(y1)),
             y = y1)) +
  geom_col() +
  geom_text(aes(label = y1), vjust = 1.5, colour = "white") # to show the numbers

Some experiments

Correct

xy %>% 
  ggplot(aes(x = fct_reorder(x1, desc(y1)),
             y = y1)) +
  geom_col() +
  scale_y_continuous(minor_breaks = seq(0, 10, .5)) +
  # scale_y_continuous(labels = scales::number_format(accuracy = .5)) +
  geom_text(aes(label = y), vjust = 1.5, colour = "white")

But then

Incorrect

xy %>% 
  ggplot(aes(x = fct_reorder(x1, desc(y1)),
             y = y1)) +
  geom_col() +
  scale_y_continuous(minor_breaks = seq(0, 10, .5)) +
  scale_y_continuous(labels = scales::number_format(accuracy = .5)) +
  geom_text(aes(label = y), vjust = 1.5, colour = "white")

Also incorrect

xy %>%
  ggplot(aes(x = fct_reorder(x1, desc(y1)),
             y = y1)) +
  geom_col() +
  scale_y_continuous(minor_breaks = seq(0, 10, 1),
                     labels = scales::number_format(accuracy = 1)) +
  geom_text(aes(label = y1), vjust = 1.5, colour = "white")

Answer 1

This is a rounding error caused by your scales_y_continuous call. Use

ggplot(xy,aes(x = fct_reorder(x1, desc(y1)),
           y = y1)) +
  geom_col() +
  scale_y_continuous(breaks=c(0,2,5,8,10)) +
  geom_text(aes(label = y1), vjust = 1.5, colour = "white")

To get what you want.

Answer 2

That 2 in yaxis is 2.5 and near 1 is 1.25 not 1.

xy %>% 
  ggplot(aes(x = fct_reorder(x1, desc(y1)),
             y = y1)) +
  geom_col() +
  scale_y_continuous(breaks = seq(0,10,2)) +
  geom_text(aes(label = y1), vjust = 1.5, colour = "white")

  xy %>% 
    ggplot(aes(x = fct_reorder(x1, desc(y1)),
               y = y1)) +
    geom_col()

I don't know why you add accuracy = 1 but take a look at plot below.

xy %>% 
  ggplot(aes(x = fct_reorder(x1, desc(y1)),
             y = y1)) +
  geom_col() +
  scale_y_continuous(breaks = seq(0,10,2))