r - ggplot2: add line for average per group

Question

library(ggplot2)

orderX <- c("A" = 1, "B" = 2, "C" = 3)
y <- rnorm(20)
x <- as.character(1:20)
group <- c(rep("A", 5), rep("B", 7), rep("C", 5), rep("A", 3))
df <- data.frame(x, y, group)
df$lvls <- as.numeric(orderX[df$group])

ggplot(data = df, aes(x=reorder(df$x, df$lvls), y=y)) + 
geom_point(aes(colour = group)) + 
geom_line(stat = "hline", yintercept = "mean", aes(colour = group))

I want to create a graph like this:

This does work, when I do not need to reorder the values of X, however, when I do use reorder, it doesn't work anymore.

Answer 1

From your question, I don't this df$x is relevant to your data at all, especially if you can re-order it. How about just using group as x, and jitter the actual x position to separate the points:

ggplot(data=df, aes(x=group,y=y,color=group)) + geom_point() +
geom_jitter(position = position_jitter(width = 0.4)) +
geom_errorbar(stat = "hline", yintercept = "mean",
  width=0.8,aes(ymax=..y..,ymin=..y..))

I have used errorbar instead of h_line (and collapsed the ymax and ymin to y) since hline is complex. If anyone has a better solution to that part, I'd love to see.

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update

If you want to preserve the order of X, try this solution (with modified X)

df$x = factor(df$x)

ggplot(data = df, aes(x, y, group=group)) + 
facet_grid(.~group,space="free",scales="free_x") + 
geom_point() + 
geom_line(stat = "hline", yintercept = "mean")