Fyi, a workaround to get what you want would be to superpose the individual area plots:
x = [0 0 0; 1 3 5; 4 8 7; 10 10 10]
y = [0 0 0; 7 8 9; 0 0 0; 0 0 0]
colors = [:red :green :blue]
p = plot()
[areaplot!(x, y, c=c, fa=0.3, st=:steppost) for (x,y,c) in zip(eachcol(x),eachcol(y),colors)]
p
