Plot only one side/half of the violin plot

There's a neat solution by @David Robinson (original code is from his gists and I did only a couple of modifications).

He creates new layer (GeomFlatViolin) which is based on changing width of the violin plot:

data <- transform(data, 
                  xmaxv = x,
                  xminv = x + violinwidth * (xmin - x))

This layer also has width argument.

---

Example:

# Using OPs data
# Get wanted width with: geom_flat_violin(width = 1.5)
ggplot(dframe, aes(group, val)) +
    geom_flat_violin()

Code:

library(ggplot2)
library(dplyr)


"%||%" <- function(a, b) {
  if (!is.null(a)) a else b
}

geom_flat_violin <- function(mapping = NULL, data = NULL, stat = "ydensity",
                        position = "dodge", trim = TRUE, scale = "area",
                        show.legend = NA, inherit.aes = TRUE, ...) {
  layer(
    data = data,
    mapping = mapping,
    stat = stat,
    geom = GeomFlatViolin,
    position = position,
    show.legend = show.legend,
    inherit.aes = inherit.aes,
    params = list(
      trim = trim,
      scale = scale,
      ...
    )
  )
}

GeomFlatViolin <-
  ggproto("GeomFlatViolin", Geom,
          setup_data = function(data, params) {
            data$width <- data$width %||%
              params$width %||% (resolution(data$x, FALSE) * 0.9)

            # ymin, ymax, xmin, and xmax define the bounding rectangle for each group
            data %>%
              group_by(group) %>%
              mutate(ymin = min(y),
                     ymax = max(y),
                     xmin = x - width / 2,
                     xmax = x)
          },

          draw_group = function(data, panel_scales, coord) {
            # Find the points for the line to go all the way around
            data <- transform(data, 
                              xmaxv = x,
                              xminv = x + violinwidth * (xmin - x))

            # Make sure it's sorted properly to draw the outline
            newdata <- rbind(plyr::arrange(transform(data, x = xminv), y),
                             plyr::arrange(transform(data, x = xmaxv), -y))

            # Close the polygon: set first and last point the same
            # Needed for coord_polar and such
            newdata <- rbind(newdata, newdata[1,])

            ggplot2:::ggname("geom_flat_violin", GeomPolygon$draw_panel(newdata, panel_scales, coord))
          },

          draw_key = draw_key_polygon,

          default_aes = aes(weight = 1, colour = "grey20", fill = "white", size = 0.5,
                            alpha = NA, linetype = "solid"),

          required_aes = c("x", "y")
)

Package see has also a function geom_violinhalf that seems to do exactly what you want (see right plot below). It behaves mostly like geom_violin(), except does not have all arguments geom_violin() has (missing for example draw_quantiles)

library(ggplot2)
library(see)


p <- ggplot(mtcars, aes(factor(cyl), mpg))
p1 <- p + geom_violin()+ ggtitle("geom_violin")
p2 <- p + see::geom_violinhalf()+ ggtitle("see::geom_violinhalf")

## show them next to each other
library(patchwork)
p1+p2

^{Created on 2020-04-30 by the reprex package (v0.3.0)}