reverse stacking order

[corybrunson]

The default ordering of strata and lodes (bottom-to-top) disagrees visually with the side legend (top-to-bottom). The stacking in ggalluvial is less straightforward than in geom_bar(), so that position_stack() is not invoked. Instead, stat_stratum() and stat_alluvium() (and any related stat layers) should admit a reverse parameter, defaulting to true, that reverses the ordering of strata and lodes.

[corybrunson]

This is underway, beginning with a standardization of the internal ordering processes, which currently look and work very differently from each other. They'll all invoke dplyr::arrange() so that a reverse parameter can be invoked easily using dplyr::desc.

[corybrunson]

Done, but without the full dplyr treatment. Issue will remain open until then.

[corybrunson]

Settling for a partial dplyr treatment.