ggplot transparency on individual bar

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I am currently attempting to use ggplot to create a bar chart with a single bar that is partially transparent.

I have the following code:

dt1 <- data.table(yr=c(2010,2010,2011,2011),
                  val=c(1500,3000,2000,1100),
                  x=c("a","b","a","b"))


ggplot() + geom_bar(data=dt1, aes(x=yr, y=val,fill=x),stat="identity") +
  scale_x_continuous(breaks=dt1$yr)

This will create a simple chart with 2 columns with stacked data. I have tried the following code to adjust the 2011 value to have transparency, however I am not having much luck. Any pointers?

dt1[,alphayr:=ifelse(yr==2011,.5,1)]
ggplot() + geom_bar(data=dt1, aes(x=yr, y=val,fill=x),stat="identity", alpha=dt1$alphayr) +
scale_x_continuous(breaks=dt1$yr)

asked 36 secs ago

Dan

237

ggplot transparency on individual bar