How to set different width values in geom_split_violin?

Question

It is known that the split violin plot could be plotted using geom_split_violin (Split violin plot with ggplot2).

# Package
library("ggplot2")

# Function
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, ...))
}

# 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))
)

# Figure1
ggplot(my_data, aes(x, y, fill = m)) + geom_split_violin(width = 1)

Here I want to set different width values for left-half and right-half violins, respectively (e.g., 0.2 and 1). Here is my attempt:

# Figure 2
ggplot(my_data, aes(x, y, fill = m)) + geom_split_violin(width = c(0.2, 1))

Hopefully, the left-half violins will be smaller than those in Figure1 and the right-half violins will be the same as those in Figrue1.

What's your desired outcome? Width values manually set for each group? Proportional to number of observations? Etc.? — Z.Lin, Dec 22 '20 at 02:58
@Z.Lin I want to set different width values for left-half and right-half violins, respectively. The `scale` parameter is `area` (default). — Feng Tian, Dec 22 '20 at 03:12
Do you mean you want to set one width value for ALL left half & another width value for ALL the right-halves? Are these values supposed to be set manually, determined from the data, etc.? It would be helpful to edit your question with an illustration of your desired end result, as well as any attempt you've made towards it. — Z.Lin, Dec 22 '20 at 04:21
@Z.Lin Yes, I would like to set one width value for ALL the left half & another width value for ALL the right half. — Feng Tian, Dec 22 '20 at 06:16

score 3 · Accepted Answer · answered Dec 22 '20 at 10:48

I added a new parameter to the code below, as a relative scale multiplier to the existing density heights:

# Function
GeomSplitViolin <- ggproto(
  "GeomSplitViolin", GeomViolin, 
  draw_group = function(self, data, rel.scale, ..., draw_quantiles = NULL) {

    grp <- data[1, "group"]
    rel.scale <- rel.scale / max(rel.scale) # rescale to (0, 1] range
    rel.scale <- rel.scale[ifelse(grp %% 2 == 1, 1, 2)] # keep only first OR second part of relative scale

    data <- transform(data, 
                      xminv = x - violinwidth * (x - xmin) * rel.scale,  # apply scale multiplier
                      xmaxv = x + violinwidth * (xmax - x) * rel.scale)
    
    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, rel.scale = c(1, 1)) {
  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, 
                      rel.scale = rel.scale, ...))
}

Usage (image shown for the first plot only):

ggplot(my_data, aes(x, y, fill = m)) +
  geom_split_violin(rel.scale = c(0.2, 1))

# equivalent to above
ggplot(my_data, aes(x, y, fill = m)) +
  geom_split_violin(rel.scale = c(1, 5))

# multipler can be applied on top of existing scale / width parameters
ggplot(my_data, aes(x, y, fill = m)) +
  geom_split_violin(scale = "count", rel.scale = c(1, 5))
ggplot(my_data, aes(x, y, fill = m)) +
  geom_split_violin(width = 0.5, rel.scale = c(1, 5))

How to set different width values in geom_split_violin?

1 Answers1