Colouring points by factor within the margin of a faceted ggplot2 plot in R

Question

I\'d like to create a faceted plot with margins in ggplot2. However, I\'d like the margin plot to have colours according to from which facet the particular point has been de

Answer 1

Another option would be to create the faceted plots and margin plot separately and use the gridExtra library to combine them:

library(ggplot2)
library(gridExtra)
mtcars$ALL <- "all"
p <- ggplot(mtcars, aes(mpg, wt))
p1 <- p + geom_point() + facet_grid(.~gear)
p2 <- p + geom_point(aes(color=factor(gear))) + facet_grid(.~ALL)
grid.arrange(p1, p2, ncol=2)

Answer 2

How about creating a new variable as a reference and colour the points by that? Seems to work provided you don't mind the points in the first 3 facets being coloured too.

mtcars$ref <- as.factor(mtcars$gear)

p <- ggplot(mtcars, aes(mpg, wt)) + geom_point(aes(col=as.factor(gear)))
p + facet_grid(.~ref, margins = TRUE)

EDIT: I have managed to get it to remove the colour key from the first 3 facets but not without playing around with the original data;

Duplicate the original data (so there are 2 of each record) then instead of using a margin plot to produce the "all" facet, using the redundant records instead.

library(ggplot2)

mtcars$ref <- (mtcars$gear)

# create the duplicate
dat <- do.call("rbind", replicate(2, mtcars, simplify = FALSE)) 

# give the duplicates a false value for "gear" so they can be plotted together 
#This value can then be used for faceting, grouping everything with "all".

dat$ref[1:32] <- "all" 


# where not in the "all" facet, change "gear" to one (so they are plotted with the same colour)
dat$gear[dat$ref != "all"] <- 1

# then plot using ref as the facet and gear to colour points.

p <- ggplot(dat, aes(mpg, wt)) + geom_point(aes(col=as.factor(gear)))
p + facet_grid(.~ref, margins = F)

I'm not sure that's the best way to go about it but perhaps someone with more expertise might be able to advise?