TODO: nested facets nesting line as element

[teunbrand]

Instead of doing the following:

ggplot(...) +
  geom_xyz(...) +
  facet_nested(
    x ~ y, nest_line = TRUE
  )

It would be nice if we can do this:

ggplot(...) +
  geom_xyz(...) +
  facet_nested(
    x ~ y, nest_line = element_line()
  )

Wherein the line element inherits from theme(ggh4x.facet.nestline).

[teunbrand]

Done: https://teunbrand.github.io/ggh4x/articles/Facets.html#nesting-lines-1