dodge or jitter only at overlaps in ggplot2

Question

I wold like to make a plot in ggplot2 with the values aligned to the center but dodged at the overlaps, like this one (done in graphpad prism)

.

What I can do in ggplot2 with jitter looks like this

df<-data.frame(response=c(-0.3502294,0.4207441,0.1001638,-0.2401336,-0.2604142,0.4574286,
       0.755964,0.9241669,0.8212376,2.037581,0.6440635,0.2714898,1.433149,0.4627742,
       0.5639637,0.1610219,0.1516505,-1.322015,-0.2134711,0.8554756,0.400872,1.344739,
       0.3743637,0.6329151,0.1467015,0.6313575,0.3989693,0.1940468,-0.06594919,-0.1951204),
    group=c(rep("A",10),rep("B",10),rep("C",10)))

set.seed(1234)
ggplot(df,aes(group,response,fill=group))+
  geom_point(size=5,pch=21,position=position_jitter(w=.2))+
  scale_y_continuous(limits=c(-2,3))

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How can I keep the points aligned to the center and dodge only at the overlaps in ggplot2?

Answer 1

My solution for your problem is to:

1. Divide data into overlapping and non-overlapping points. This can be done by calculating the difference between previous points. If it is less than some threshold, then that point is overlapping with some point and so some zitter should be applied, while plotting that point. Otherwise the point is plotted as such.
2. Plot the two data separately using geom_point.

Below is the code, using above logic.

library(data.table)

threshold <- 0.1

df<-data.frame(response=c(-0.3502294,0.4207441,0.1001638,-0.2401336,-0.2604142,0.4574286,
       0.755964,0.9241669,0.8212376,2.037581,0.6440635,0.2714898,1.433149,0.4627742,
       0.5639637,0.1610219,0.1516505,-1.322015,-0.2134711,0.8554756,0.400872,1.344739,
       0.3743637,0.6329151,0.1467015,0.6313575,0.3989693,0.1940468,-0.06594919,-0.1951204),
    group=c(rep("A",10),rep("B",10),rep("C",10)))

df1 <- data.table(df[order(df$group, df$response),])
df1[, diffFromLast:=response - shift(response, n=1, type="lag"), by=group]
nonZitterPoints <- df1[ is.na(diffFromLast) | (diffFromLast > threshold),]
zitterPoints <- df1[ which(diffFromLast < threshold),]

g1 <- ggplot()+
       geom_point(data=nonZitterPoints,aes(group,response,fill=group), size=5,pch=21)+
       scale_y_continuous(limits=c(-2,3))
g1

g2 <- g1 + geom_point(data=zitterPoints,aes(group,response,fill=group), size=5,pch=21, position=position_jitter(w=.2))

g2