Sort ribbon/area in setup_data

[thomasp85]

This PR streamlines the handling of data in line-like gloms where an implicit sorting of data along the x-axis is happening. Previously GeomLine had the sorting happening in setup_data while GeomRibbon (and by extension GeomArea) had it inside draw_group.

The motivation is that it solves some issues in gganimate that couldn't be fixed outside of ggplot2, but I also think that conceptually this is the right approach, and we should be consistent in how we treat data operations across kindred geoms/stats

[clauswilke]

Looks good to me. It might cause some downstream effects though for any third-party geoms that inherit from GeomArea and expect the old behavior, so maybe make a note of this change in NEWS.md.

[thomasp85]

Yeah, maybe... I think in the worst case they just do an additional sort on an already sorted data.frame, but who knows... I'll add a note

[clauswilke]

Actually, if they implement their own setup_data() function but use the draw_group() function provided by GeomRibbon then they'll draw unsorted data. Just like you had to fix setup_data() for GeomArea.

[thomasp85]

That is true. The note is there anyway, but I don’t expect any problems

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