I'd like to create a split violin density plot using ggplot, like the fourth example on this page of the seaborn documentation.
Here is some data:
set.seed(20160229)
my_data = data.frame(
y=c(rnorm(1000), rnorm(1000, 0.5), rnorm(1000, 1), rnorm(1000, 1.5)),
x=c(rep('a', 2000), rep('b', 2000)),
m=c(rep('i', 1000), rep('j', 2000), rep('i', 1000))
)
I can plot dodged violins like this:
library('ggplot2')
ggplot(my_data, aes(x, y, fill=m)) +
geom_violin()
But it's hard to visually compare the widths at different points in the side-by-side distributions. I haven't been able to find any examples of split violins in ggplot - is it possible?
I found a base R graphics solution but the function is quite long and I want to highlight distribution modes, which are easy to add as additional layers in ggplot but will be harder to do if I need to figure out how to edit that function.
Or, to avoid fiddling with the densities, you could extend ggplot2
's GeomViolin like this:
GeomSplitViolin <- ggproto("GeomSplitViolin", GeomViolin,
draw_group = function(self, data, ..., draw_quantiles = NULL) {
data <- transform(data, xminv = x - violinwidth * (x - xmin), xmaxv = x + violinwidth * (xmax - x))
grp <- data[1, "group"]
newdata <- plyr::arrange(transform(data, x = if (grp %% 2 == 1) xminv else xmaxv), if (grp %% 2 == 1) y else -y)
newdata <- rbind(newdata[1, ], newdata, newdata[nrow(newdata), ], newdata[1, ])
newdata[c(1, nrow(newdata) - 1, nrow(newdata)), "x"] <- round(newdata[1, "x"])
if (length(draw_quantiles) > 0 & !scales::zero_range(range(data$y))) {
stopifnot(all(draw_quantiles >= 0), all(draw_quantiles <=
1))
quantiles <- ggplot2:::create_quantile_segment_frame(data, draw_quantiles)
aesthetics <- data[rep(1, nrow(quantiles)), setdiff(names(data), c("x", "y")), drop = FALSE]
aesthetics$alpha <- rep(1, nrow(quantiles))
both <- cbind(quantiles, aesthetics)
quantile_grob <- GeomPath$draw_panel(both, ...)
ggplot2:::ggname("geom_split_violin", grid::grobTree(GeomPolygon$draw_panel(newdata, ...), quantile_grob))
}
else {
ggplot2:::ggname("geom_split_violin", GeomPolygon$draw_panel(newdata, ...))
}
})
geom_split_violin <- function(mapping = NULL, data = NULL, stat = "ydensity", position = "identity", ...,
draw_quantiles = NULL, trim = TRUE, scale = "area", na.rm = FALSE,
show.legend = NA, inherit.aes = TRUE) {
layer(data = data, mapping = mapping, stat = stat, geom = GeomSplitViolin,
position = position, show.legend = show.legend, inherit.aes = inherit.aes,
params = list(trim = trim, scale = scale, draw_quantiles = draw_quantiles, na.rm = na.rm, ...))
}
And use the new geom_split_violin
like this:
ggplot(my_data, aes(x, y, fill = m)) + geom_split_violin()
Note : I think the answer by jan-glx is much better, and most people should use that instead.
You can achieve this by calculating the densities yourself beforehand, and then plotting polygons. See below for a rough idea.
library(dplyr)
pdat <- my_data %>%
group_by(x, m) %>%
do(data.frame(loc = density(.$y)$x,
dens = density(.$y)$y))
pdat$dens <- ifelse(pdat$m == 'i', pdat$dens * -1, pdat$dens)
pdat$dens <- ifelse(pdat$x == 'b', pdat$dens + 1, pdat$dens)
ggplot(pdat, aes(dens, loc, fill = m, group = interaction(m, x))) +
geom_polygon() +
scale_x_continuous(breaks = 0:1, labels = c('a', 'b')) +
ylab('density') +
theme_minimal() +
theme(axis.title.x = element_blank())
It is now possible to do this with the introdataviz package using the geom_split_violin function, which makes it really easy to create these plots. Here is a reproducible example:
set.seed(20160229)
my_data = data.frame(
y=c(rnorm(1000), rnorm(1000, 0.5), rnorm(1000, 1), rnorm(1000, 1.5)),
x=c(rep('a', 2000), rep('b', 2000)),
m=c(rep('i', 1000), rep('j', 2000), rep('i', 1000))
)
library(ggplot2)
# devtools::install_github("psyteachr/introdataviz")
library(introdataviz)
ggplot(my_data, aes(x = x, y = y, fill = m)) +
geom_split_violin()
Created on 2022-08-24 with reprex v2.0.2
As you can see, it creates a split violin plot. If you want more information and a tutorial of this package, check the link above.
@jan-jlx's solution is wonderful. For densities with thin tails, I'd like to insert a little space between the two halves of the violin so the tails are easier to tell apart. Here's a slight modification of @jan-jlx's code to do this, borrowing the nudge parameter from the gghalves package:
GeomSplitViolin <- ggplot2::ggproto(
"GeomSplitViolin",
ggplot2::GeomViolin,
draw_group = function(self,
data,
...,
nudge = 0,
draw_quantiles = NULL) {
data <- transform(data,
xminv = x - violinwidth * (x - xmin),
xmaxv = x + violinwidth * (xmax - x))
grp <- data[1, "group"]
newdata <- plyr::arrange(transform(data,
x = if (grp %% 2 == 1) xminv else xmaxv),
if (grp %% 2 == 1) y else -y)
newdata <- rbind(newdata[1, ],
newdata,
newdata[nrow(newdata), ],
newdata[1, ])
newdata[c(1, nrow(newdata)-1, nrow(newdata)), "x"] <- round(newdata[1, "x"])
newdata$x <- ifelse(newdata$group %% 2 == 1,
newdata$x - nudge,
newdata$x + nudge)
if (length(draw_quantiles) > 0 & !scales::zero_range(range(data$y))) {
stopifnot(all(draw_quantiles >= 0), all(draw_quantiles <= 1))
quantiles <- ggplot2:::create_quantile_segment_frame(data,
draw_quantiles)
aesthetics <- data[rep(1, nrow(quantiles)),
setdiff(names(data), c("x", "y")),
drop = FALSE]
aesthetics$alpha <- rep(1, nrow(quantiles))
both <- cbind(quantiles, aesthetics)
quantile_grob <- ggplot2::GeomPath$draw_panel(both, ...)
ggplot2:::ggname("geom_split_violin",
grid::grobTree(ggplot2::GeomPolygon$draw_panel(newdata, ...),
quantile_grob))
}
else {
ggplot2:::ggname("geom_split_violin",
ggplot2::GeomPolygon$draw_panel(newdata, ...))
}
}
)
geom_split_violin <- function(mapping = NULL,
data = NULL,
stat = "ydensity",
position = "identity",
nudge = 0,
...,
draw_quantiles = NULL,
trim = TRUE,
scale = "area",
na.rm = FALSE,
show.legend = NA,
inherit.aes = TRUE) {
ggplot2::layer(data = data,
mapping = mapping,
stat = stat,
geom = GeomSplitViolin,
position = position,
show.legend = show.legend,
inherit.aes = inherit.aes,
params = list(trim = trim,
scale = scale,
nudge = nudge,
draw_quantiles = draw_quantiles,
na.rm = na.rm,
...))
}
Here's a plot I get with geom_split_violin(nudge = 0.02)
.
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