Overlay violin plots ggplot2

Question

I would like to plot two series of ten violin plots one over the second :

library(ggplot2)
#generate some data

coco1<-rnorm(10000,0,1)
coco2<-c(runif(10000))
decile<-rbinom(10000,9,1/2)+1
coconut<-data.frame(coco1,coco2,decile)

#draw the violin plots of the coco1 serie
p <- ggplot(coconut, aes(factor(decile), coco1))
p<-p + geom_violin(aes(alpha=0.3,colour="#1268FF"))
p

#draw the violin plots of the coco2 serie
q <- ggplot(coconut, aes(factor(decile), coco2))
q<-q + geom_violin(aes(alpha=0.3,colour="#3268FF"))
q

I would like to plot the violin plot "p" and "q", on the same graph, and I want each violin plot of "q" to be over the corresponding violin plot of "p".

Answer 1

You can just add the geom_violin of the second plot to your first one:

p <- ggplot(coconut, aes(factor(decile), coco1))
p <- p + geom_violin(aes(colour = "#1268FF"), alpha = 0.3)
q <- p + geom_violin(aes(y = coco2, colour = "#3268FF"), alpha = 0.3)

Now, q contains both versions of the violins.

By the way: If you want to get rid of the colour legend, you have to specify the colour outside of aes.

Answer 2

Another way is to set position to be "identity". This example also uses tidyr::gather to move the data.frame around a bit to make it easier to plot in ggplot.

library(ggplot2)

coco1<-rnorm(10000,0,1)
coco2<-c(runif(10000))
decile<-rbinom(10000,9,1/2)+1
coconut<-tidyr::gather(
    data.frame(coco1,coco2,decile),
    name,value,coco1,coco2)

p <- ggplot(coconut)+
    aes(x=factor(decile),y=value,color=name)+
    geom_violin(alpha=0.3,position="identity")
p

ggsave("example_identity.png",p)