element_text() ignores left and right margins for some plot elements

[ccsarapas]

Left and right margins get ignored for some elements (including plot titles and captions), but work fine for other elements (such as strip and axis labels.

Given this plot:

library(ggplot2)

p <- ggplot(mpg, aes(cty, hwy)) +
  geom_point() +
  facet_wrap(vars(cyl)) +
  labs(
    title = "City vs. Highway Mileage by Cylinders", 
    caption = "Values are in MPG."
  ) +
  theme(
    plot.background = element_rect(color = "black", linewidth = 1),
    axis.title.y = element_text(angle = 0, vjust = 0.5),
    strip.text = element_text(hjust = 0)
  )

p

You can change the top and bottom margins for the title and caption, but the left and right margins don't budge:

p + 
  theme(
    plot.title = element_text(margin = margin(20, 20, 20, 20)),
    plot.caption = element_text(margin = margin(20, 20, 20, 20))
  )

But all 4 margins seem to work for axis titles and strip labels:

p +
  theme(
    strip.text = element_text(margin = margin(20, 20, 20, 20)),
    axis.title.y = element_text(margin = margin(20, 20, 20, 20))
  )

^{Created on 2023-11-22 with reprex v2.0.2}

[teunbrand]

For some theme elements, only one margin is used purposefully, see for example the (sub)title/caption declarations here:

ggplot2/R/plot-build.R

Lines 267 to 277 in ca47270

	# Title
	title <- element_render(theme, "plot.title", plot$labels$title, margin_y = TRUE)
	title_height <- grobHeight(title)
	# Subtitle
	subtitle <- element_render(theme, "plot.subtitle", plot$labels$subtitle, margin_y = TRUE)
	subtitle_height <- grobHeight(subtitle)
	# whole plot annotation
	caption <- element_render(theme, "plot.caption", plot$labels$caption, margin_y = TRUE)
	caption_height <- grobHeight(caption)

For some elements, say x-axis text, the horizontal margins are ignored due to the labels being linked to a single point that varies in the x-direction. This makes sense to me.

Other elements are linked to a cell in the layout, like the titles. It makes less sense to me to ignore part of the margins there.
The default theme's margins are 0 for left/right (sub)titles and captions, so I don't see any explicit harm in enabling them, but I'd like to wait out to hear what the others think.