How to insert grouped median segments in violin plot in ggplot2 - r

Question

I'd like to insert median lines for factor levels into a violin plot in ggplot2. Here's some reproducible data:
set.seed(12)
FactorVar <- sample(LETTERS[1:5], 500, replace = T)
NumericVar <- abs(rnorm(500))
df <- data.frame(FactorVar, NumericVar)
To get the grouped medians I use tapply:
medians <- tapply(df$NumericVar, df$FactorVar, FUN = median)
And this is the code for the plot. As can be seen, I'm inserting each median line individually. That's cumbersome and uneconomical:
library(ggplot2)
g <-
ggplot(data = df,
aes(x = FactorVar, y = NumericVar, fill = FactorVar)) +
geom_violin(scale = "count", trim = F, adjust = 0.75) +
geom_point(aes(y = NumericVar),
position = position_jitter(width = .15), size = 0.9, alpha = 0.8) +
geom_hline(yintercept = mean(NumericVar), color = "blue", size = 0.8, linetype = 4) +
geom_segment(x = 0.5, xend = 1.5, y= medians[1], yend = medians[1], color = "red", linetype = 2) +
geom_segment(x = 1.5, xend = 2.5, y = medians[2], yend = medians[2], color = "red", linetype = 2) +
geom_segment(x = 2.5, xend = 3.5, y = medians[3], yend = medians[3], color = "red", linetype = 2) +
geom_segment(x = 3.5, xend = 4.5, y = medians[4], yend = medians[4], color = "red", linetype = 2) +
geom_segment(x = 4.5, xend = 5.5, y = medians[5], yend = medians[5], color = "red", linetype = 2) +
guides(fill = FALSE) +
guides(color = FALSE) +
coord_flip() +
theme_gray(); g
How can the median segments be inserted in a single command? Also, observe how the median line for factor A is thinner than the others? Why's that?

Answer 1

One method (that simplifies the +/- axis) would be to facet it. Before, though, we'll need to put the medians into a frame, preferably with the same grouping factors as the original.
mediansdf <- data.frame(FactorVar=names(medians), NumericVar=medians)
g <-
ggplot(data = df,
aes(x = FactorVar, y = NumericVar, fill = FactorVar)) +
geom_violin(scale = "count", trim = F, adjust = 0.75) +
geom_point(aes(y = NumericVar),
position = position_jitter(width = .15), size = 0.9, alpha = 0.8) +
geom_hline(yintercept = mean(NumericVar), color = "blue", size = 0.8, linetype = 4) +
guides(fill = FALSE) +
guides(color = FALSE) +
coord_flip() +
theme_gray() +
facet_grid(FactorVar~., scales="free") +
geom_segment(aes(x = 0.5, xend = 1.5, yend = NumericVar), color = "red", linetype = 2, data = mediansdf)
g
This example reused the y aesthetic, but since we have a different frame, we could easily use different names (and specify them within aes(...). One advantage to using the same variable names is (in my opinion) clearer declarative code.
Since the facet_grid adds the factor label on the right side, you likely could remove it from the axis. Note, if you do not use scales="free", then you'll see all factors in each facet, which is distracting and unnecessary.
The reason I am suggesting facets is that it makes the x and xend simple and relative to a single violin, so 0.5 to 1.5; otherwise, as you saw, there is some assumption on which is going with which integer placement.
Last, the appearance of thinner red lines for me was while looking at the raster plot window. If you save to vector-based format (e.g., PDF), the lines appear to be the same thickness.