如何堆叠以下示例中的图形?
a<-as.POSIXlt("2013-07-01 00:00:00",origin = "1960-01-01",tz="GMT")
b<-as.POSIXlt("2013-07-08 00:00:00",origin = "1960-01-01",tz="GMT")
woche1<-sample(seq(as.numeric(a),by=60*60,length.out=200),200,T)
woche2<-sample(seq(as.numeric(b),by=60*60,length.out=200),200,T)
times<-c(woche1,woche2)
class(times)<-c("POSIXt","POSIXct")
times<-as.POSIXlt(times,origin = "1960-01-01",tz="GMT")
key<-sample(LETTERS[1:3],200,T)
df<-data.frame(times=times,order=factor(rep(1:2,each=100)), key=key)
p<-ggplot(df, aes(x=times))
p<-p + geom_area(aes(y = ..count.., fill = key, group = key),stat = "bin",position = 'stack')#,position = 'stack'
p<-p + facet_wrap( ~ order,scales="free_x")
p