position_stack(vjust = 1, reverse = FALSE)

position_fill(vjust = 1, reverse = FALSE)

# Stacking and filling ------------------------------------------------------

# Stacking is the default behaviour for most area plots.
# Fill makes it easier to compare proportions
ggplot(mtcars, aes(factor(cyl), fill = factor(vs))) +
  geom_bar()
ggplot(mtcars, aes(factor(cyl), fill = factor(vs))) +
  geom_bar(position = "fill")

ggplot(diamonds, aes(price, fill = cut)) +
  geom_histogram(binwidth = 500)
ggplot(diamonds, aes(price, fill = cut)) +
  geom_histogram(binwidth = 500, position = "fill")

# Stacking is also useful for time series
series <- data.frame(
  time = c(rep(1, 4),rep(2, 4), rep(3, 4), rep(4, 4)),
  type = rep(c('a', 'b', 'c', 'd'), 4),
  value = rpois(16, 10)
)
ggplot(series, aes(time, value)) +
  geom_area(aes(fill = type))

# Stacking order ------------------------------------------------------------
# The stacking order is carefully designed so that the plot matches
# the legend.

# You control the stacking order by setting the levels of the underlying
# factor. See the forcats package for convenient helpers.
series$type2 <- factor(series$type, levels = c('c', 'b', 'd', 'a'))
ggplot(series, aes(time, value)) +
  geom_area(aes(fill = type2))

# You can change the order of the levels in the legend using the scale
ggplot(series, aes(time, value)) +
  geom_area(aes(fill = type)) +
  scale_fill_discrete(breaks = c('a', 'b', 'c', 'd'))

# If you've flipped the plot, use reverse = TRUE so the levels
# continue to match
ggplot(series, aes(time, value)) +
  geom_area(aes(fill = type2), position = position_stack(reverse = TRUE)) +
  coord_flip() +
  theme(legend.position = "top")

# Non-area plots ------------------------------------------------------------

# When stacking across multiple layers it's a good idea to always set
# the `group` aesthetic in the ggplot() call. This ensures that all layers
# are stacked in the same way.
ggplot(series, aes(time, value, group = type)) +
  geom_line(aes(colour = type), position = "stack") +
  geom_point(aes(colour = type), position = "stack")

ggplot(series, aes(time, value, group = type)) +
  geom_area(aes(fill = type)) +
  geom_line(aes(group = type), position = "stack")

# You can also stack labels, but the default position is suboptimal.
ggplot(series, aes(time, value, group = type)) +
  geom_area(aes(fill = type)) +
  geom_text(aes(label = type), position = "stack")

# You can override this with the vjust parameter. A vjust of 0.5
# will center the labels inside the corresponding area
ggplot(series, aes(time, value, group = type)) +
  geom_area(aes(fill = type)) +
  geom_text(aes(label = type), position = position_stack(vjust = 0.5))

# Negative values -----------------------------------------------------------

df <- tibble::tribble(
  ~x, ~y, ~grp,
  "a", 1,  "x",
  "a", 2,  "y",
  "b", 1,  "x",
  "b", 3,  "y",
  "b", -1, "y"
)
ggplot(data = df, aes(x, y, group = grp)) +
  geom_col(aes(fill = grp), position = position_stack(reverse = TRUE)) +
  geom_hline(yintercept = 0)

ggplot(data = df, aes(x, y, group = grp)) +
  geom_col(aes(fill = grp)) +
  geom_hline(yintercept = 0) +
  geom_text(aes(label = grp), position = position_stack(vjust = 0.5))