overlapping plots are not sized correctly

[sebkopf]

When overlapping plots, they are not sized correctly, e.g. the below has plots 1 & 2 completely overlapping and plot 3 not at all and as a result plots 1 & 2 are much larger than plot 3

library(ggstackplot)
ggstackplot(
  mtcars,
  x = mpg,
  y = c(wt, qsec, drat),
  color = c("#E41A1C", "#377EB8", "#4DAF4A"),
  overlap = c(1, 0)
)