library(ggplot2)
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, ...))
}

######################

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))
)
head(my_data)
##            y x m
## 1  1.4197928 a i
## 2 -1.1848608 a i
## 3  1.4152642 a i
## 4 -0.2112585 a i
## 5  0.5247534 a i
## 6  0.9622425 a i
str(my_data)
## 'data.frame':    4000 obs. of  3 variables:
##  $ y: num  1.42 -1.185 1.415 -0.211 0.525 ...
##  $ x: chr  "a" "a" "a" "a" ...
##  $ m: chr  "i" "i" "i" "i" ...
my_data$m <- factor(my_data$m)
ggplot(my_data, aes(x, y, fill=m)) + 
  geom_split_violin(trim = TRUE) + 
  geom_boxplot(width = 0.25, notch = FALSE, notchwidth = .4, 
               outlier.shape = NA, coef=0, position=position_dodge(0.8)) +
  labs(x=NULL,y="GM Attitude Score") +
  theme_classic() +
  theme(text = element_text(size = 20)) +
  scale_x_discrete(labels=c("0" = "Control\nCondition", "1" = "GM\nCondition")) +
  scale_fill_manual(values=c("#E69F00", "#999999"), 
                    name="Survey\nPart",
                    breaks=c("i", "j"),
                    labels=c("Time 1", "Time 5"))

##ref https://gist.github.com/Karel-Kroeze/746685f5613e01ba820a31e57f87ec87
##ref https://stackoverflow.com/questions/51228076/ggplot-split-violin-plot-with-horizontal-mean-lines
##ref https://mp.weixin.qq.com/s/U1STWy4eO_htb4-TSroujQ