Draw lines connecting individual data points in grouped bar plot - ggplot

Question

I have a grouped bar plot (two conditions with two groups in each). I am not having trouble creating the group bar plots, but I am interested in putting individual data points on the plot and connecting them with a line. More specifically, the lines should be connected within conditions, so the data point from condition 1 group 1 connects to the corresponding data point in condition 1 group 2. I cannot find a way to do this without using facet_wrap.

I have seen facet_wrap being suggested here, but it affects the alignment of the plots when using patchwork specifically. I have provided some code for the bar plots and the code to produce the facet_wrap plot.

I am open to any suggestions which do not use facet_wrap. I was hoping there would be a way to do it directly using geom_line(aes(subject)), but that does not seem to be grouping correctly.

Code

df <- data.frame(   
    Subject = c(1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3),   
    Condition = c(“A”, “A”, “B”, “B”, “A”, “A”, “B”, “B”, “A”, “A”, “B”, “B”),   
    Group = c(“group1”, “group2", “group1”, “group2",                
              “group1”, “group2", “group1”, “group2",                
              “group1”, “group2", “group1”, “group2"),   
    Value = c(10, 7, 8, 9, 11, 12, 13, 2, 2, 3, 4, 5))

Desired Look (excluding the connected data points)

ggplot(df, aes(x=Condition, y=Value, fill=Group)) +   
    stat_summary(fun.data = mean_sdl, geom = ‘bar’, position = ‘dodge’) +   
    stat_summary(fun.data = mean_se, geom = ‘errorbar’, width = 0.2, 
                 position = position_dodge(width = 0.9)) +   
    theme_minimal()

Connected Data Points (using undesired facet)

ggplot(df, aes(x=Group, y=Value, fill=Group)) +   
    stat_summary(fun.data = mean_sdl, geom = ‘bar’, position = ‘dodge’) +   
    stat_summary(fun.data = mean_se, geom = ‘errorbar’, width = 0.2, 
                 position = position_dodge(width = 0.9)) +   
    theme_minimal() +   
    geom_line(aes(group = Subject), alpha = 0.2, position = position_nudge(c(.14, -.14))) +
    facet_wrap(~Condition)

Here is an example of how facet affects the alignment in patchwork (image on the right). Because it assumes there should be an axis title everything is misaligned.

Answer 1

Your group aesthetic in geom_line() needs to incorporate both the Subject and Condition information for this to work without faceting. Try the following solution (I have also inverted the signs in the position_nudge() call, since as they were they were inverting your data points, and have added a parameter nudgeAmount to easily control the nudge and a geom_point() call so you can see where the actual data points are):

nudgeAmount <- 0.14
ggplot(df, aes(x = Condition, y = Value, fill = Group)) +   
    stat_summary(fun.data = mean_sdl, geom = "bar", position = "dodge") +   
    stat_summary(fun.data = mean_se, geom = "errorbar", width = 0.2, position = position_dodge(width = 0.9)) +
    geom_point(shape = 21, position = position_nudge(nudgeAmount*c(-1,1))) +
    geom_line(aes(group = paste(Subject,Condition)), alpha = 0.2, position = position_nudge(nudgeAmount*c(-1,1))) +
    theme_minimal()