Control color of legend elements that are not colour guides in ggplot

Question

When using a ggplot2 theme with a black background, can one control the legend colour for guides other than the colour guide so that things are not plotted in black? If so, how?

library(ggplot2) # needs to be 0.9.3 for this theme
data(iris)       # included with ggplot2

theme_black<- function (base_size = 16, base_family = ""){
    theme_minimal() %+replace% 
        theme(
              line = element_line(colour = "white", size = 0.5, linetype = 1, 
                        lineend = "butt"), 
              rect = element_rect(fill = "white", 
                        colour = "white", size = 0.5, linetype = 1), 
              text = element_text(family = base_family, 
                        face = "plain", colour = "white", size = base_size,
                        angle = 0, lineheight = 0.9, hjust = 0, vjust = 0),
              plot.background = element_rect(colour = 'black', fill = 'black'),
              plot.title = element_text(size = rel(1.2)),
              panel.border = element_rect(fill = NA, colour = "white"), 
              panel.grid.major = element_line(colour = "grey20", size = 0.2), 
              panel.grid.minor = element_line(colour = "grey5", size = 0.5),
              strip.background = element_rect(fill = "grey30", colour = "grey30")
             )
    }

ggplot(data=iris, aes(x=Sepal.Length, y=Sepal.Width, shape=Species,
       colour=Petal.Length))+geom_point()+theme_black()+
       scale_colour_gradient(low = "purple", high = "white")

As you can see, the default colour for the shape part of the legend has not been changed, so it is invisible and one cannot tell which species is which:

The only solution I have right now is to change the legend.background colour, but this is a waste of ink and ugly.

Answer 1

The short answer seems to be that there is no perfect way to do this.

We can add hidden layers with shape guides as per @user1317221_G's answer, which I accepted. But it is extra computation and if we save as pdf, I expect these hidden layers will be present.

Alternatively, we can override the shape guide's colour, as @bdemarest suggests in comments:

+ guides(shape=guide_legend(override.aes=list(colour="white"))

But this still forces us to add theme-specific code beyond just the +theme_black()

I've implemented a slightly more elaborate version of this because I set a default theme for the session depending on whether I am making plots for paper (grey/white background) or screen (black background). So my approach is to run something along these lines early in the session:

 set_theme(theme_black); defaultcol = "white" # for slides
 # or
 set_theme(theme_bw); defaultcol = "black"    # for paper

followed by a ggplot() that includes this:

+ guides(shape=guide_legend(override.aes=list(colour=defaultcol))

This has the advantage of minimizing the need for theme-specific adjustments to plots, though it is not as good as being able to control the default ggplot2 colour with a theme.

Answer 2

One way would be to add two extra geom_points, the logic would be:

plot white points for the legend, cover them with black points with no legend, then plot your coloured points with no legend,e.g.

geom_point(colour="white",size=1) + 
geom_point(colour="black",size=3,show_guide=FALSE) +
geom_point(show_guide=FALSE)