ggplot2: add line for average per group

26
votes
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: graph with averages for each group

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.

2
rggplot2
I think your use of reorder is mistaken here, since it will just reorder X, not groups or Y. This will plot the wrong x with the wrong y! – Alex Brown
Unless X doesn't mean anything but index, in which case, don't use it in the plot (use jitter instead?) – Alex Brown
Then my use of reorder is mistaken. In my real data the values on x are labels for each individual measurement, which I do want to see. The ordering of these labels within the groups does not matter. – wligtenberg
Maybe another reason why it does not work in my case is, because my x-values are not numeric, but character. – wligtenberg
+1 for a concise question, with sample data and a picture. I'd give +1 for each of those if I could. – Alex Brown

2 Answers

18
votes

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.

alt text

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")

alt text

7
votes

As of ggplot2 2.x this approach is unfortunately broken.

The following code provides exactly what I wanted, with some extra calculations up front:

library(ggplot2)
library(data.table)

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))
dt <- data.table(x, y, group)
dt[, lvls := as.numeric(orderX[group])]
dt[, average := mean(y), by = group]
dt[, x := reorder(x, lvls)]
dt[, xbegin := names(which(attr(dt$x, "scores") == unique(lvls)))[1], by = group]
dt[, xend := names(which(attr(dt$x, "scores") == unique(lvls)))[length(x)], by = group]

ggplot(data = dt, aes(x=x, y=y)) + 
    geom_point(aes(colour = group)) +
    facet_grid(.~group,space="free",scales="free_x") + 
    geom_segment(aes(x = xbegin, xend = xend, y = average, yend = average, group = group, colour = group))

The resulting image:

enter image description here