Support stacked area plots

[grantmcdermott]

Something like:

plt(
   Freq ~ Dept | Admit, data = as.data.frame(UCBAdmissions),
   facet = ~ Gender, facet.args = list(ncol = 1), frame = FALSE,
   type = type_area(stack = TRUE),
   theme = "float"
)

[zeileis]

Good idea! Two quick comments:

1. facet = Gender ~ 1 is now the easier way to express facet = ~ Gender, facet.args = list(ncol = 1).
2. If you put that example into the documentation, it might be useful to also add the corresponding spineplot which shows the marginal distribution of Dept as well.

ucb <- as.data.frame(UCBAdmissions)
plt(
   Admit ~ Dept, data = ucb,
   facet = Gender ~ 1, frame = FALSE,
   type = type_spineplot(weights = ucb$Freq),
   theme = "float"
)