Mirror one side of a population pyramid

position_pyramid() draws the rows of one side in the negative direction, so that the two sides of a pyramid extend away from a shared age axis, and stacks subgroups within each side. It works with any layer, so bars, labels, points and lines all land on the same side.

Usage

position_pyramid(left = NULL, stack = TRUE, vjust = 1, reverse = FALSE)

Arguments

left: The value of the side variable drawn on the left in this layer. Overrides the left of coord_pyramid(), which sets the side for every layer at once. Without either, the first level is drawn on the left when the side variable is a factor, and the first value in sorted order otherwise.
stack: If TRUE, rows that share an age group and a side are stacked away from the centre, as in ggplot2::position_stack(). If FALSE, they overlap, which suits a comparison layer drawn over the bars. Ranges that do not start at zero, such as error bars, are never stacked.
vjust, reverse: Passed on to the stacking. See ggplot2::position_stack().

Value

A ggproto object of class PositionPyramid.

Aesthetics

position_pyramid() understands the side aesthetic, which assigns each row to one of the two sides. When side is not mapped, the side comes from fill, and failing that from colour. Map side explicitly when fill shows subgroups, for example aes(fill = region, side = sex). Rows with a missing side are drawn on the right.

Signed values

The data you supply stays positive. Inside the plot, values on the left side are negative, so a manual reference line or limit on that side carries a minus sign: geom_vline(xintercept = -100) or scale_x_continuous(limits = c(-500, 500)). coord_pyramid() shows the axis labels without the sign; with another coordinate system or in a vertical pyramid, use scale_x_pyramid() or scale_y_pyramid().

Orientation

The axis that does not hold age is the one that gets mirrored. Age is the discrete axis when only one of them is discrete, and the y axis when both are continuous, so a pyramid is horizontal unless age is mapped to x. A mapped hjust (or vjust in a vertical pyramid) is mirrored as well, so text justified away from the bar end on the right does the same on the left. A fixed hjust set outside ggplot2::aes() is not mirrored.

Examples

library(ggplot2)

pop <- data.frame(
  age = factor(rep(c("0-19", "20-39", "40-59", "60+"), 2)),
  sex = factor(rep(c("Male", "Female"), each = 4), c("Male", "Female")),
  n = c(220, 200, 160, 90, 210, 205, 170, 120)
)

# Any geom can be mirrored
ggplot(pop, aes(x = n, y = age, fill = sex)) +
  geom_col(position = position_pyramid()) +
  geom_text(aes(label = n, hjust = -0.2), position = "pyramid") +
  scale_x_continuous(expand = expansion(mult = 0.15)) +
  coord_pyramid()


# Subgroups stack within each side
pop2 <- rbind(transform(pop, group = "A"), transform(pop, group = "B"))
ggplot(pop2, aes(x = n, y = age, fill = group, side = sex)) +
  geom_col(position = "pyramid") +
  coord_pyramid(left = "Female")