R ggplot transparency - alpha values conditional on other variable

Question

I have the following data: [example from R graphics Cookbook]

Cultivar Date Weight sd          n  se          big
c39     d16   3.18  0.9566144   10  0.30250803          


        

Answer 1

The problem here is that your variable is discrete, whereas the alpha scale is continuous. One way to do it is to manually compute your alpha values before plotting :

alpha <- ifelse(d$big, 0.9, 0.35)
ggplot(d, aes(x=Date, y=Weight, fill=Cultivar)) +
    geom_bar(position="dodge", stat="identity", aes(alpha=alpha))

The downside is that you won't get a correct legend for your alpha values. You can delete it with this :

ggplot(d, aes(x=Date, y=Weight, fill=Cultivar)) +
    geom_bar(position="dodge", stat="identity", aes(alpha=big)) +
    scale_alpha_continuous(guide=FALSE)

Final result :

Answer 2

Another possibility using scale_alpha_discrete, where the range argument may be used to set your desired alpha values for each level of 'big'.

ggplot(data = cabbage_exp, aes(x = Date, y = Weight, fill = Cultivar, alpha = big)) +
  geom_bar(position = "dodge", stat = "identity") +
  scale_alpha_discrete(range = c(0.35, 0.9))