Minimal PR for unit support in positional aesthetics

[mjskay]

Following discussion on #5609 and helpful feedback from @teunbrand, @pmur002, and @thomasp85 (thanks!), this is a second proof-of-concept PR for unit support in positional aesthetics in ggplot2. This PR focuses on being a minimal implementation without hacking on the unit type system.

The basic idea here is to instead add another aesthetic evaluation stage, after_coord, which is akin to after_stat or after_scale. It is implemented by passing aesthetic mappings with stage() or after_coord() through to Coord$transform() via the panel_params argument, which is already passed through to that function. Coord$transform() can then apply the after_coord mappings after it does its own Coord-specific transformations.

Some demos:

data.frame(var1 = 1:5, var2 = 1:5, name = letters[1:5]) |>
  ggplot(aes(var1, var2)) + 
  geom_point() +
  geom_line() +
  # labels exactly 8 points left and 6 points above their points, no matter how 
  # the plot is resized
  geom_text(aes(
    label = name, 
    x = stage(var1, after_coord = unit(x, "native") - unit(8, "pt")),
    y = stage(var2, after_coord = unit(y, "native") + unit(6, "pt"))
  )) +
  # an annotation that is always 10 points inset from the lower right
  # N.B. can't use annotate() because it doesn't understand stages, but using a 
  # geom with data = data.frame() instead seems to work. If desired, I believe
  # annotate() could be made to understand stages by having it check for the
  # stage / after_stat / after_scale /after_coord functions in the bare
  # expressions supplied to it.
  geom_text(
    aes(
      x = after_coord(unit(1, "npc") - unit(10, "pt")),
      y = after_coord(unit(10, "pt"))
    ),
    label = "some label", vjust = 0, hjust = 1,
    data = data.frame()
  ) +
  # a horizontal line exactly 25 points from the bottom.
  # can't use after_coord() with the yintercept aesthetic because geom_hline()
  # translates that to `y` and `yend` under the hood before the after_coord()
  # stage, so must modify those values directly
  geom_hline(aes(
    yintercept = Inf,  # placeholder value to avoid "missing required aes" error
    y = after_coord(unit(25, "pt")),
    yend = after_coord(unit(25, "pt")),
  ))

With polar coordinates:

data.frame(var1 = 1:5, var2 = 1:5, name = letters[1:5]) |>
  ggplot(aes(var1, var2)) + 
  geom_point() +
  geom_line() +
  geom_text(aes(
    label = name, 
    x = stage(var1, after_coord = unit(x, "native") + unit(10, "pt"))
  )) +
  geom_text(
    aes(
      x = after_coord(unit(1, "npc") - unit(10, "pt")),
      y = after_coord(unit(10, "pt"))
    ),
    label = "some label", vjust = 0, hjust = 1,
    data = data.frame()
  ) +
  coord_polar()

Some notes:

• The main workhorse here is compute_staged_aes(), which is based on a chunk of code I factored out of Geom$use_defaults() for computing after_scale aesthetics.

• after_coord() is a bit different from after_scale() in that it provides the placeholder value Inf to earlier parts of the pipeline, because it needed a non-unit() value that would not affect scales (see the comment on that function). There should probably be a better way to do this.

• the implementation moves current Coord$transform() implementations into Coord$transform_numeric(). This was done so that Geoms (which already call Coord$transform()) would not have to opt in to supporting unit()s; instead, Coords in extension packages can opt-in simply by renaming their transform() methods to transform_numeric(). An alternative might be to leave Coord$transform() as-is but add a Coord$transform_unit() method, and then have Geoms opt-in to unit support instead of having Coords opt-in to it. I think having Coords opt-in is better since it requires fewer changes in extension packages and propagates support more easily, but I could be wrong.

• No hacking on unit() is required for this implementation, nor any upstream changes to {grid}. A small set of compatibility functions would need to be added to {vctrs}, akin to how {vctrs} supplies compatibility functions for other base-R types, like Dates. These are confined to R/utilities-unit.R, which could be turned into a PR on {vctrs}.

• In exchange for less "magic", this approach is a little more verbose than my original proposal and requires a little more understanding of the ggplot2 pipeline for users. Something like this in the current proposal:

  x = stage(var1, after_coord = unit(x, "native") - unit(8, "pt"))

  could be written as this in the original proposal:

  x = var1 - ggunit(8, "pt")

  However, this new proposal has more consistent semantics and no heuristic hacking of the unit() datatype :). A ggunit() subtype of unit() with consistent coercion rules for numerics could still be created to improve the syntax a bit (also without heuristic hacking of unit()); allowing something like this to work:

  x = stage(var1, after_coord = x - ggunit(8, "pt"))

  Adding shortcuts like as_pt() could shorten this still, to:

  x = stage(var1, after_coord = x - as_pt(8))

  The ggunit() subtype could be added to ggplot2, or if that is not desired, I'd be happy to create a small extension package with that capability.