Stacked area charts not stacking

[karawoo]

When I try to create a stacked area chart using the following code the segments don't stack, and they occlude each other instead.

library("ggplot2")
library("dplyr")

## Create data
set.seed(40)
dat <- data.frame(x = rep(c(1:10), 3),
                  var = rep(c("a", "b", "c"), 10),
                  y = round(runif(30, 1, 5)))

## Plot
ggplot(dat, aes(x = x, y = y, fill = var)) +
  geom_area(position = "stack")

However if I reorder the rows so all of the var values are grouped together, it does create a stacked chart:

dat %>%
  arrange(var, x) %>%
  ggplot(aes(x = x, y = y, fill = var)) +
  geom_area(position = "stack")

Is this a bug, or is the ordering of the rows meant to matter like this?

[thomasp85]

It sort of makes weird sense as the order at each x-value follows the order it occurs in in the data. I would suggest we add a sorting step to position_stack - In this way we can also get the stacking order to follow the legend order...

@hadley thoughts? It might change the appearance of some plots, though I would argue that the benefit of the bug fix and sorting would alleviate that...

[hadley]

I like the idea of matching the stacking order to the legend order. That would also help with #1593

[thomasp85]

It would not be bulletproof, i.e. it is only possible to match the stacking to the default legend order and not to any modified order defined in scale_*, that is, we can sort only by the data itself as position doesn't have access to the scale object...

[hadley]

Oh that's true - and it would mean that to change the stacking order you'd be forced to change the levels of a factor. Hmmm.

[thomasp85]

I think this is reasonable. If well documented it is not too much to ask that they reorder factor levels to achieve their desired stacking...