r - Fill being ignored with group + facet_wrap in ggplot2 / geom_bar

Question

I suspect I might be using group incorrectly here, but I can't seem to understand why the fill color is getting ignored in the example below.

df <- data.frame(a = factor(c(1,1,2,2,1,2,1,2)), 
                 b = factor(c(1,2,3,4,5,6,7,2)), 
                 c = factor(c(1,2,1,2,1,2,1,2)))
p <- ggplot(df, aes(x=b)) +
  geom_bar(aes(y =  ..density.., group = c, fill=a), binwidth = 1) + 
  facet_wrap(~ c) +
  scale_y_continuous(labels = percent_format()) +
  scale_color_hue()
p

Any help would be greatly appreciated. Thanks in advance, --JT

Answer 1

I think I understand what plot you're after now. I'd do something like this:

df <- data.frame(a = c(1,1,2,2,1,2,1,2), 
             b = c(1,2,3,4,5,6,7,2), 
             c = c(1,2,1,2,1,2,1,2))

df <- within(df, { f <- 1 / ave(b, list(c), FUN=length)})
df[, 1:3] <- lapply(df[, 1:3], as.factor)

ggplot(df, aes(x = b)) + geom_bar(stat = "identity", position = "stack", 
           aes(y = f, group = c, fill = a), binwidth = 1) + facet_wrap(~ c) + 
           scale_y_continuous(labels = percent_format())

This gives the plot: