This bug was probably introduced by f2f5e8c, which was a fix for #622.
This should have lines connecting the means:
ggplot(mtcars, aes(cyl, hp, colour = factor(gear))) + geom_point() +
stat_summary(aes(group = factor(gear)), fun.y = mean, geom = "line")
# geom_path: Each group consist of only one observation. Do you need to adjust the group aesthetic?
same as #732 ?
Revert stat_summary grouping. Fixes #739 and #732