Why won't the ggplot2 legend combine manual fill and scale values?

Question

Expected Behavior

If I create a plot with ggplot2 and use separate, say, shape and fill scales to delineate data, I would expect the legend would delineate between "white" filled points (which look hollow) and "black" filled points (which do not look hollow).

In the example code below, the legend item for Windows should be a white hollow point, and that for Linux should be a black filled in point

Actual Behavior

The legend items under "Operating System" depict two visually identical points for what are clearly different operating systems whose points are clearly drawn with different fills on the graph. In the sample code below, both Windows and Linux appear as indistinguishable black hollow points in the legend, even though they're properly plotted differently on the plot itself.

Sample Plot

Sample Code

library(ggplot2)

x <- rnorm(n = 30)
y <- rnorm(n = 30)
treatment <- rep(c("red", "green", "blue"), times = 20)
operatingSystem <- rep(c("Windows", "Linux"), times = 30)

dd <- data.frame(x, y, treatment, operatingSystem)

fillScaleValues <- c(
  "Windows" = "white",
  "Linux" = "black"
)

shapeScaleValues <- c(
  "red" = 21,
  "green" = 22,
  "blue" = 23
)

p <- ggplot(
      aes(x = x, 
          y = y,
          shape = factor(treatment),
          fill = factor(operatingSystem)
      ), data = dd
     )

p <- p + geom_point()
p <- p + scale_fill_manual(values = fillScaleValues, name = "Operating System")
p <- p + scale_shape_manual(values = shapeScaleValues, name = "Treatment")

p

Session Info

R version 2.15.1 (2012-06-22)
Platform: x86_64-apple-darwin9.8.0/x86_64 (64-bit)

locale:
[1] C/en_US.UTF-8/C/C/C/C

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

other attached packages:
[1] ggplot2_0.9.2.1       reshape2_1.2.1        plyr_1.7.1            ProjectTemplate_0.4-2
[5] testthat_0.7         

loaded via a namespace (and not attached):
 [1] MASS_7.3-21        RColorBrewer_1.0-5 colorspace_1.1-1   dichromat_1.2-4   
 [5] digest_0.5.2       evaluate_0.4.2     grid_2.15.1        gtable_0.1.1      
 [9] labeling_0.1       memoise_0.1        munsell_0.4        proto_0.3-9.2     
[13] scales_0.2.2       stringr_0.6.1      tools_2.15.1

Marius Marius · Accepted Answer · 2012-09-19T06:30:44

You have to override the shape that's being used in the legend, as seen in this question.

So using your example code (thanks for the clear, reproducible question, by the way), all you need to do is:

p + guides(fill = guide_legend(override.aes = list(shape = 21)))

Which gives you what you wanted:

Proper legend