Manually create a legend when you cannot deliver color aesthetics

Question

In an attempt to answer this question, one way to create the desired plot was to use geom_dotplot from ggplot2 as follows:

 library(ggplot2) library(reshape2) CTscores <- read.csv(text="initials,total,interest,slides,presentation CU,1.6,1.7,1.5,1.6 DS,1.6,1.7,1.5,1.7 VA,1.7,1.5,1.5,2.1 MB,2.3,2.0,2.1,2.9 HS,1.2,1.3,1.4,1.0 LS,1.8,1.8,1.5,2.0") CTscores.m = melt(CTscores, id.var="initials") ggplot(CTscores.m, aes(x=variable, y=value)) + geom_dotplot(binaxis="y", stackdir="up",binwidth=0.03) + theme_bw()+coord_flip() 

To distinguish points, it would be convenient to simply add color, but geom_dotplot throttled by color and does not finish their styling:

 ggplot(CTscores.m, aes(x=variable, y=value, fill=initials)) + geom_dotplot(binaxis="y", stackdir="up",binwidth=0.03,color=NA) + theme_bw()+coord_flip() 

Color can be added manually using a hack, though:

 gg_color_hue <- function(n) { hues = seq(15, 375, length=n+1) hcl(h=hues, l=65, c=100)[1:n] } cols <- rep(gg_color_hue(6),4) ggplot(CTscores.m, aes(x=variable, y=value)) + geom_dotplot(binaxis="y", stackdir="up",binwidth=0.03,fill=cols,color=NA) + theme_bw()+coord_flip() 

Unfortunately, there is no legend. In addition, we cannot use aes(fill=) to try to add the legend manually, because it will hide the dots. Is there a way to add a legend without using aes() ?

Answer 1

Using the gtable package gtable you can extract a legend from a graph with a legend that cannot add points and add this legend with grid.arrange from the gridExtra package to the plot with colored ans folded points as follows:

 p1 <- ggplot(CTscores.m, aes(x=variable, y=value)) + geom_dotplot(binaxis="y", stackdir="up", binwidth=0.03, fill=cols, color=NA) + coord_flip() + theme_bw() p2 <- ggplot(CTscores.m, aes(x=variable, y=value, fill=initials)) + geom_dotplot(binaxis="y", stackdir="up", binwidth=0.03, color=NA) + coord_flip() + theme_bw() library(gtable) fill.legend <- gtable_filter(ggplot_gtable(ggplot_build(p2)), "guide-box") legGrob <- grobTree(fill.legend) library(gridExtra) grid.arrange(p1, legGrob, ncol=2, widths = c(4,1)) 

which gives: