使用ggplot2 scale_colour_discrete稳定映射：drop不起作用？

Question

当使用ggplot并尝试为不同图中的类别设置稳定的颜色映射时，如何使drop=TRUE工作（因此图例仅包含子集中存在的类别）scale_colour_discrete？

此问题与this一个问题相关联，尤其是this评论。

在链接问题中从one of the answers借来的可重现代码：

set.seed(2014)
library(ggplot2)
dataset <- data.frame(category = rep(LETTERS[1:5], 100),
                      x = rnorm(500, mean = rep(1:5, 100)),
                      y = rnorm(500, mean = rep(1:5, 100)))
dataset$fCategory <- factor(dataset$category)
subdata <- subset(dataset, category %in% c("A", "D", "E"))
ggplot(dataset, aes(x = x, y = y, colour = fCategory)) + geom_point()
ggplot(subdata, aes(x = x, y = y, colour = fCategory)) + geom_point() + 
       scale_colour_discrete(drop=TRUE,limits = levels(dataset$fCategory))

为什么drop=TRUE在第二个情节中不起作用？图例仍包含所有类别。

sessionInfo()的输出：

R version 3.1.2 (2014-10-31)
Platform: x86_64-w64-mingw32/x64 (64-bit)

locale:
[1] LC_COLLATE=English_United Kingdom.1252  LC_CTYPE=English_United     Kingdom.1252   
[3] LC_MONETARY=English_United Kingdom.1252 LC_NUMERIC=C                           
[5] LC_TIME=English_United Kingdom.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] ggplot2_1.0.0

loaded via a namespace (and not attached):
[1] colorspace_1.2-4 digest_0.6.8     grid_3.1.2       gtable_0.1.2         labeling_0.3    
[6] MASS_7.3-35      munsell_0.4.2    plyr_1.8.1       proto_0.3-10         Rcpp_0.11.3     
[11] reshape2_1.4.1   scales_0.2.4     stringr_0.6.2    tools_3.1.2     

Answer 1

这可能是对drop所做的错误概念（不幸的是，帮助条目没有提供太多细节）或错误。不过，我建议完全删除drop（双关语）并同时设置limits和breaks：

ggplot(subdata, aes(x = x, y = y, colour = fCategory)) + geom_point() + 
  scale_colour_discrete(limits = levels(dataset$fCategory), 
                        breaks = unique(subdata$fCategory))

颜色设置一致，图例很好。