Additional facet.args for prefixing, etc.

[grantmcdermott]

Closes #295

(as well as some additional, related functionality.)

This PR adds three new facet.args arguments:

• prefix: for adding an informative prefix to facet titles a la Option to prefix facet titles with facet variable name #295.
• labeller: for formatting facet titles via tinylabel().
• sep: for controlling how the individual variables of a multi-variable facet title
  are separated, e.g. use sep = "\n" to stack on separate lines.

Quick MWEs

pkgload::load_all("~/Documents/Projects/tinyplot/")
#> ℹ Loading tinyplot
plt(mpg ~ wt, data = mtcars, facet = ~vs) ## base case: not very informative

plt(
  mpg ~ wt, data = mtcars,
  facet = ~vs,
  facet.args = list(prefix = TRUE)
)

plt(
  mpg ~ wt, data = mtcars,
  facet = ~vs,
  facet.args = list(
    prefix = TRUE,
    labeller = as.logical
  )
)

Special care is given over to multi-variable facet, in which case both prefix and labeller accept (named) lists to prepend/format each variable separately. Here's a kitchen sink example:

plt(
  mpg ~ wt, data = mtcars,
  facet = ~am:vs,
  facet.args = list(
    prefix = list("vs" = "V-shape", "am" = "Automatic"),
    labeller = as.logical,
    sep = "\n"
  ),
  theme = "clean"
)

^{Created on 2026-08-20 with reprex v2.1.1}

[jangorecki]

This is nice! Thanks