geom_bar()/geom_col() erroneously warn that they ignore width aesthetic

[richierocks]

geom_bar() and geom_col() let you specify a width aesthetic to control the width of the bars.

The behavior is as expected, but it generates an erroneous warning "Warning: Ignoring unknown aesthetics: width".

width isn't listed in the aesthetics section of ?geom_bar, so it appears that this is an unofficial behavior.

---

library(ggplot2)
suppressPackageStartupMessages(library(dplyr))
mtcars_by_cyl <- mtcars %>% 
  group_by(cyl) %>% 
  summarize(
    mean_wt = mean(wt),
    n = n()
  ) %>% 
  mutate(prop = n / sum(n))

ggplot(mtcars_by_cyl) + 
  geom_col(aes(cyl, mean_wt, width = prop))
#> Warning: Ignoring unknown aesthetics: width

Created on 2019-02-13 by the reprex package (v0.2.0).

---

Similar closed issues.

• Regression with width aesthetic in documentation #1904
• Ignoring unknown aesthetics "width" for geom_col() #2473

[yutannihilation]

Currently, width is recognized as a parameter by a "hack". Here's the comment written 4 years ago. Maybe it's worth trying to make width to a proper aes?

ggplot2/R/geom-bar.r

Line 130 in 43dcd63

# Hack to ensure that width is detected as a parameter

[ptoche]

elsewhere (e.g. boxplot), width is added to the list of extra_params:

ggplot2/R/geom-boxplot.r

Line 162 in 03bd946

# need to declare `width`` here in case this geom is used with a stat that

[clauswilke]

width works just fine as a parameter in the way the code is currently written, and the "hack" is fine also. The question is whether width should be an aesthetic. I'm skeptical, because bars with varying widths are not normally meaningful. It's not that different a case from bars that start from a base value other than zero, which we also don't support. If people really want to do something like this, they can use geom_rect() or geom_tile() instead.

[yutannihilation]

The question is whether width should be an aesthetic

Isn't width already an aes? At least, the plot above seems to have varying widths of bars.

[yutannihilation]

Sorry, I was confused. Now I come to think the varying widths of geom_col() is just a mistake. It uses data$width, but it should be really "ignored" as the warning says.

ggplot2/R/geom-col.r

Lines 40 to 46 in 43dcd63

	setup_data = function(data, params) {
	data$width <- data$width %||%
	params$width %||% (resolution(data$x, FALSE) * 0.9)
	transform(data,
	ymin = pmin(y, 0), ymax = pmax(y, 0),
	xmin = x - width / 2, xmax = x + width / 2, width = NULL
	)

In geom_bar()'s case, stat_count() provides the width, so it should be used. But, geom_col() uses stat_identity(), which we should not expect width.

[yutannihilation]

But, in terms of the interface (I don't mean the current behaviour is semantically correct), width is provided by a Stat via data. So, it is virtually an aes.

I'm wondering why width is not passed via param...

[yutannihilation]

Oh, this last example reminds me of the need for varying width.

# You can specify a function for calculating binwidth,
# particularly useful when faceting along variables with
# different ranges

https://ggplot2.tidyverse.org/reference/geom_histogram.html

[yutannihilation]

Here's my understanding. Is this correct?

• We want to enforce a constant bar width within a panel, so width cannot be an aes.
• Yet, the width can vary among panels, so we need to pass widths per bar via data, not a single value via param.
• data$width should be used only when the Stat provides it. But, geom_col() is not the case, it should ignore data$width.

[ptoche]

Just a comment in passing: If width were to be passed as an aes to capture the relative amounts of some variable in the dataset, the bar-chart would become a sort of rectangular-shaped pie-chart, where the area --- not the length --- becomes the relevant metric (I don't think that's "meaningless", but would suffer from most of the problems that pie-charts have). As far as I can tell, Hadley (among others) is not fond of pie-charts.

For the standard bar-chart with "meaningless" width, I would argue that the current default width of geom_bar is too wide : narrower bars would help the eye focus on the important metric --- height. Excel and LibreOffice/Calc seem to go for a default of 100%, i.e. the space between bars = width of the bars. geom_bar is wider than that. Anyone else thinks it ought to be narrower?

library("reprex")

library("ggplot2")
ggplot(mtcars, aes(x = gear)) + geom_bar()

ggplot(mtcars, aes(x = gear)) + geom_bar(width = 0.5)

ggplot(mtcars, aes(x = gear)) + geom_bar(width = 0.25)

^{Created on 2019-02-18 by the reprex package (v0.2.1)}

[mattansb]

Just to comment here that allowing width as an aesthetic can be used to have different "sized" pies in pie charts, which is quite useful (I mean, as useful as a pie chart can be...):

library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
library(ggplot2)

d <- mtcars %>% 
  group_by(am) %>% 
  count(cyl) %>% 
  mutate(total = sum(n),
         norm_n = n / total)

(p <- ggplot(d, aes(0, norm_n, fill = factor(cyl))) + 
    facet_grid(cols = vars(am)) + 
    geom_col(aes(width = total), position = position_stack()))
#> Warning: Ignoring unknown aesthetics: width

p + aes(x = total/2) + coord_polar("y")

^{Created on 2021-08-12 by the reprex package (v2.0.0)}