Telling ggplot NOT to scale alpha

问题

I would like to use the explicit values for the alpha level.

head(D)

    x  y group  alpha
  1 1 18     A   0.40   <~~~~
  2 2 18     A   0.44
  3 3 18     A   0.47
  4 1 18     A   0.51
  5 2 21     B   0.55
  6 3 21     B   0.58
  ...

However, ggplot is scaling the alpha levels. I can override this using scale_alpha_continuous(range=range(D$alpha)), but this becomes a neusance when creating the graph programatically.

Is there a direct way to tell ggplot NOT to scale alpha? (instead of telling it what range to scale to)

Reproducible Exmple

library(ggplot)
library(gridExtra)
(D <- data.frame(x=rep(1:3, 4), y=rep((6:8)*3, each=4), group=rep(c("A","B", "C"), each=4),  alpha=round(seq(.4, .8, length.out=12), 2)))

P <- ggplot(data=D, aes(x=x, y=y, alpha=alpha)) + geom_bar(stat="identity", fill="blue") + theme(legend.position="bottom") + facet_grid(group ~. )

### Adding  scale_alpha_continuous
P.manually_scaled <- P + scale_alpha_continuous(range=range(D$alpha))

grid.arrange( P + ggtitle("INCORRECT")
             , P.manually_scaled + ggtitle("CORRECT")
             , ncol=2)

回答1:

If you have actual alpha, color, ..., values then you should use ..identity() scales. This will tell ggplot() to assign alpha values as they are in your data frame and not to scale them.

ggplot(data=D, aes(x=x, y=y, alpha=alpha)) + 
         geom_bar(stat="identity", fill="blue") + 
         facet_grid(group ~. ) +
         scale_alpha_identity()

来源：https://stackoverflow.com/questions/22408820/telling-ggplot-not-to-scale-alpha

标签

ggplot2

alpha