library(ggplot2)
set.seed(1)
df <- data.frame(
  x=rep(sort(sample(1:20, 5)), 3),
  y=ave(runif(15), rep(1:3, each=5), FUN=cumsum),
  grp=letters[rep(1:3, each=5)]
)
ggplot(df, aes(x=x, y=y, color=grp)) + geom_step(position="stack")

df2 <- rbind(
  df,
  transform(df[order(df$x),],
    x=x - 1e-9,  # required to avoid crazy steps
    y=ave(y, grp, FUN=function(z) c(z[[1]], head(z, -1L)))
) )
ggplot(df2, aes(x=x, y=y, fill=grp)) + geom_area()