Is there a way to add superscript and subscript to the same element using ggtext,<sub>&<sup>?

Question

I have a necessary reason to use 'ggtext'

This is not the result I want,You can see that there is a big gap between '2-' and 'O'

library(ggtext)
library(tidyverse)
df <- tibble(
  label = c(
    'SO<sub>4</sub><sup>2-</sup>'
  ),
  x = c(.5),
  y = c(.5),
)
ggplot(df) +
  aes(
    x, y, label = label
  ) +
  geom_richtext() +
  xlim(0, 1)+ 
  ylim(0, 1)

This is the result I want, but this is not achieved with ggtext's html tags sub and sup

Answer 1

I can't find any reference that suggests HTML or markdown support the "atop" notion (aka vertical alignment of superscript and subscript); some answers have even said "not possible", though I can't say with certainty that that's the case.

This is possible using the basic geom_label and plotmath-expressions.

Since you said that you have reason to use ggtext, is it possible to separate the labels with this feature?

df <- tibble(
  label = c(
    'SO<sub>4</sub><sup>2-</sup>',
    "'SO'[4]^'2-'"
  ),
  x = c(.5, .5),
  y = c(.4, .6),
  parse = c(FALSE, TRUE)
)
ggplot(df) +
  aes( x, y, label = label ) +
  geom_richtext(data = ~ filter(., !parse)) +
  geom_label(data = ~ filter(., parse), parse=TRUE) +
  xlim(0, 1) + ylim(0, 1)

If desired, you can add some space after the SO to make it a little less cramped; for instance, "'SO '[4]^'2-'" results in