How to extract the fill colours from a ggplot object?

Question

I am trying to write some automated unit tests for a series of functions that generates ggplot graphics.

For example, I want to set a specific colour scale to a plot. Now I need a way to determine whether the correct colour scale was actually applied.

The background:

Here is some example code, that set the fill colour to use the ColourBrewer palette Dark2:

p <- ggplot(mtcars, aes(x=factor(cyl), y=mpg, fill=factor(gear))) +    geom_bar(stat="identity") +    facet_grid(~gear) +   scale_fill_brewer(palette="Dark2")  print(p) 

OK, so a visual inspection tells me the code worked.

What I've tried:

Now I want to confirm this by inspecting the object:

str(p, max.level=1) List of 8  $ data       :'data.frame':    32 obs. of  11 variables:  $ layers     :List of 1  $ scales     :Reference class 'Scales' [package "ggplot2"] with 1 fields   ..and 20 methods, of which 9 are possibly relevant  $ mapping    :List of 3  $ options    :List of 1  $ coordinates:List of 1   ..- attr(*, "class")= chr [1:2] "cartesian" "coord"  $ facet      :List of 9   ..- attr(*, "class")= chr [1:2] "grid" "facet"  $ plot_env   :<environment: R_GlobalEnv>   - attr(*, "class")= chr "ggplot" 

Fine, the $scales object seems interesting. Let's look at that in more detail:

str(p$scales) Reference class 'Scales' [package "ggplot2"] with 1 fields  $ scales:List of 1   ..$ :List of 14   .. ..$ call      : language discrete_scale(aesthetics = "fill", scale_name = "brewer", palette = brewer_pal(type, palette))   .. ..$ aesthetics: chr "fill"   .. ..$ scale_name: chr "brewer"   .. ..$ palette   :function (n)     .. ..$ range     :Reference class 'DiscreteRange' [package "scales"] with 1 fields   .. .. ..$ range: NULL   .. .. ..and 14 methods, of which 3 are possibly relevant:   .. .. ..  initialize, reset, train   .. ..$ limits    : NULL   .. ..$ na.value  : logi NA   .. ..$ expand    : list()   .. .. ..- attr(*, "class")= chr "waiver"   .. ..$ name      : NULL   .. ..$ breaks    : list()   .. .. ..- attr(*, "class")= chr "waiver"   .. ..$ labels    : list()   .. .. ..- attr(*, "class")= chr "waiver"   .. ..$ legend    : NULL   .. ..$ drop      : logi TRUE   .. ..$ guide     : chr "legend"   .. ..- attr(*, "class")= chr [1:3] "brewer" "discrete" "scale"  and 20 methods, of which 9 are possibly relevant:    add, clone, find, get_scales, has_scale, initialize, input, n, non_position_scales 

But here I draw a blank. There is nothing inside p$scales that looks like either my input palette, or in fact like colours.

What I expect to find:

The colours that I would expect are:

library(RColorBrewer) brewer.pal(3, name="Dark2") [1] "#1B9E77" "#D95F02" "#7570B3" 

The question:

How do I interrogate a ggplot object for specific fill colours to use?

Answer 1

Try building the plot,

g <- ggplot_build(p) unique(g$data[[1]]["fill"])        fill 1  #1B9E77 16 #D95F02 28 #7570B3