Jittered layering + offsets

[grantmcdermott]

Fixes #493

Jitter layers added via tinyplot_add() now align correctly with grouped boxplot, violin, and ridge base layers.

Operationalized through the new group_offset setting, which the different data_* constructor functions can access. This also means that offsets are calculated ahead of time, instead of during the drawing phase (which is probably more robust anyway).

MWEs:

pkgload::load_all('~/Documents/Projects/tinyplot')
#> ℹ Loading tinyplot

plt(
  body_mass ~ sex | species, data = penguins,
  flip = TRUE,
  type = "box",
  theme = "clean"
)
plt_add(type = "j", cex = 0.5)

plt(
  body_mass ~ sex | species, data = penguins,
  flip = TRUE,
  type = "violin",
  bg = 0.2,
  theme = "clean"
)
plt_add(type = "j", cex = 0.5)

The "ridge" layering still doesn't look great b/c the colours don't inherit from the base layer and points near the baseline may be clipped by ylim... But those are separate issues IMO.

plt(
  sex ~ body_mass, data = penguins,
  type = "ridge",
  theme = "ridge"
)
plt_add(type = "j", cex = 0.5)

^{Created on 2026-03-23 with reprex v2.1.1}