Stacked area plots
Passing type_area(stack = TRUE) stacks the by groups cumulatively, rather than drawing each one from a zero baseline. Groups are accumulated in the order of their (factor) levels, so the first level forms the bottom band and the top of the final band traces the group total. Stacking is computed separately within each facet.
The byord argument is a helpful companion to stacked area plots, since it enables on-the-fly adjustment of the stacking order. For example, the size keywords—“desc”, “start”, and “end”—rank the stacked by groups according to their y values, either summed across the x axis or taken at one end of it. Following convention, the ranking runs in descending order, so that the biggest group is drawn on the bottom layer (use “asc” for the reverse). However, size is not the only route to a stable baseline. Because each band is drawn on top of the ones below it, they all inherit whatever movement the bottom layer has. A large but volatile group can therefore be a worse choice of foundation than a small, steady one. In this latter case, the “minvar” keyword would be a more appropriate choice since it places the lowest variance group at the bottom. Your choice of stacking ordering should therefore respond to the patterns in your data and which layers you want to emphasize.
Stacking needs exactly one y value per group per x value. Repeated cells —typically caused by a variable that is present in the data but absent from the plot—are collapsed with FUN (default mean) rather than being stacked against each other. Conversely, groups that are missing an x value (or have an NA there) count as contributing zero at that point, so that a gap in one group does not shift the groups stacked above it. Note that stacking negative values is not meaningful and will produce overlapping bands. Note that the legend key for stacked area plots is deliberately inverted compared to other plot types (including non-stacked area plots) to ensure a consistent ordering with the "bottoms-up" layering of the stacked regions. Similarly, reordering of the by group levels will reassign the palette, since group colours are allocated by level position. This matches what releveling a factor does elsewhere, but it does mean that reordering the bands repaints them.
Finally, note that unlike non-stacked area plots, the stacked bands are drawn with opaque fill by default, since they do not overlap. Pass an explicit alpha or fill value to override.
