Ordering of label placement with `geom_text()` depends on whether or not a group aesthetic is set

[TimTeaFan]

When creating a stacked bar chart and the fill argument is not placed in the initial ggplot call, but in the aes of geom_bar, the label order of geom_text is displayed in reverse order. This behavior can be prevented, when adding fill to the aes of geom_text, however, this produces a warning saying ...

Ignoring unknown aesthetics: fill

... which is not really correct, since this unknown aesthetics still has some effect on the final plot.

I posted this on SO and Claus Wilke found out that the issue is related to grouping. Once the fill variable is added to geom_text(... group = ...) labels are displayed in correct order.

I see two possible issues here:

1. Is the reverse label order intended, when calling fill in the aes of geom_bar (and without adding the fill variable to group in geom_text)? Why is the default labeling in reverse order?
2. Might the warning when providing fill to aes in geom_bar be misleading, since it still has an effect on the plot?

A short example:

library(dplyr)
library(ggplot2)

# some toy data
data <- tibble(Category = c("Baseball",
                            "Basketball",
                            "Football",
                            "Hockey"),
               n = c(45,10,25,20)) 

  # fill is provided to the initial ggplot call - labels are in correct order
  ggplot(data, aes (x="", y = n, fill = Category)) +
    geom_bar(width = 1, stat = "identity") +
    geom_text(aes(label = paste(n, "%")),
            position = position_stack(vjust = 0.5))

   # fill is provided to geom_bar - labels are in reverse order
  ggplot(data, aes (x="", y = n)) +
    geom_bar(aes(fill = Category), width = 1, stat = "identity") +
    geom_text(aes(label = paste(n, "%")),
              position = position_stack(vjust = 0.5))

  # fill is provided to aes in geom_text - yields warning, but labels are in correct order
  ggplot(data, aes (x="", y = n)) +
  geom_bar(aes(fill = Category), width = 1, stat = "identity") +
  geom_text(aes(fill = Category, label = paste(n, "%")),
          position = position_stack(vjust = 0.5))
  #> Warning: Ignoring unknown aesthetics: fill

  # fill variable is provided to group in geom_text - labels are in correct order
  ggplot(data, aes (x="", y = n)) +
    geom_bar(aes(fill = Category), width = 1, stat = "identity") +
    geom_text(aes(label = paste(n, "%"), group = Category),
              position = position_stack(vjust = 0.5))

[batpigandme]

Rendered with reprex:

library(dplyr)
library(ggplot2)

# some toy data
data <- tibble(Category = c("Baseball",
                            "Basketball",
                            "Football",
                            "Hockey"),
               n = c(45,10,25,20)) 

# fill is provided to the initial ggplot call - labels are in correct order
ggplot(data, aes (x="", y = n, fill = Category)) +
  geom_bar(width = 1, stat = "identity") +
  geom_text(aes(label = paste(n, "%")),
            position = position_stack(vjust = 0.5))

# fill is provided to geom_bar - labels are in reverse order
ggplot(data, aes (x="", y = n)) +
  geom_bar(aes(fill = Category), width = 1, stat = "identity") +
  geom_text(aes(label = paste(n, "%")),
            position = position_stack(vjust = 0.5))

# fill is provided to aes in geom_text - yields warning, but labels are in correct order
ggplot(data, aes (x="", y = n)) +
  geom_bar(aes(fill = Category), width = 1, stat = "identity") +
  geom_text(aes(fill = Category, label = paste(n, "%")),
            position = position_stack(vjust = 0.5))
#> Warning: Ignoring unknown aesthetics: fill

#> Warning: Ignoring unknown aesthetics: fill

# fill variable is provided to group in geom_text - labels are in correct order
ggplot(data, aes (x="", y = n)) +
  geom_bar(aes(fill = Category), width = 1, stat = "identity") +
  geom_text(aes(label = paste(n, "%"), group = Category),
            position = position_stack(vjust = 0.5))

^{Created on 2019-11-04 by the reprex package (v0.3.0)}

[clauswilke]

To be clear: The fill aesthetic is irrelevant here, it just happens to have a side effect on the group aesthetic. The question is: Why does the behavior of geom_text() depend on whether or not there's a group aesthetic set in this example, and is this behavior as intended or not?

[yutannihilation]

I'm sure this is intended. If a data contains groups, ordering should be determined based on the group to ensure all the data is stacked in the same order at every X value.

[clauswilke]

@yutannihilation Feel free to close this issue if you think it's all as expected.

[yutannihilation]

Hmm, I'm just saying this should be intended. I agree this looks unexpected so it's nice if we can improve this. For reference, the ordering probably happens here:

ggplot2/R/position-collide.r

Lines 39 to 45 in 3870c12

	# Reorder by x position, then on group. The default stacking order reverses
	# the group in order to match the legend order.
	if (reverse) {
	data <- data[order(data$xmin, data$group), ]
	} else {
	data <- data[order(data$xmin, -data$group), ]
	}

[yb2125]

If you order data by category before plotting the chart, the error message will not appear.

[teunbrand]

Why does the behavior of geom_text() depend on whether or not there's a group aesthetic set in this example, and is this behavior as intended or not?

I don't think this is a geom_text() issue, setting a group in geom_text() might (not 100% sure) affect the drawing order, but not the position. The issue is with position_stack(). If we read the issue title as "Ordering of placement with position_stack() depends on whether or not a group aesthetic is set", then it reads as if the current behaviour is correct (which I think it is).

Unless we start sharing information between layers, which we don't on purpose, I don't think there is a lot to improve here. As such, I'll close this issue but we can always re-open if someone disagrees.