Export `default_expansion()` and `expand_limits_scale()` for coord extensions

[matthewkling]

I'd like to request that default_expansion() and expand_limits_scale() be exported (or that equivalent public functions be provided). These two internal helpers appear to be the only way for a coord extension to reproduce ggplot2's exact axis-expansion behavior.

I maintain ggcube, a coord extension that adds a 3D coordinate system (coord_3d()) to ggplot2. Because it extends CoordCartesian, it needs to expand scale limits identically to core ggplot2 — including taking a user-supplied expand argument — so that the z axis behaves like the x and y axes. To do this, the relevant ggcube code currently uses these internal functions by reaching into the ggplot2 namespace:

default_expansion   <- utils::getFromNamespace("default_expansion",   "ggplot2")
expand_limits_scale <- utils::getFromNamespace("expand_limits_scale", "ggplot2")

expansion      <- default_expansion(scale, expand = expand)
expanded_range <- expand_limits_scale(scale, expansion, limits, coord_limits = NULL)

Relying on these internals is fragile. If they were exported, it would fix that risk. ggplot2 already exports some adjacent helper functions, including expansion() and expand_limits(), so there seems to be a precedent.

Thanks for considering.

[teunbrand]

Thanks for posting the issue Matthew!

I think the conceptual issue here is that both the coord and the scale are involved with the computation. Currently, only coords have control over this, but the scale expansion is much more closely related to scales. I think it might be cleanest to have scale expansion as a scale method, which is then called by the coord. That way, the scale behaviour lives in the relevant class, and you should be able to use something like scale_x_continuous()$expand_limits() in your code without having to rely on these internals.

[matthewkling]

Thanks Teun. Yeah, refactoring to a scale$expand_limits() method would work well for my use case, and it seems like a cleaner layout in general than having this step live in the coord.