geom_path: Each group consists of only one observation. Do you need to adjust the group aesthetic?
solution
ggplot(result_prod,aes(Phase,
MeanACC,
shape=Subtype,
group=Subtype))
ggplot(result_prod,aes(Phase,
MeanACC,
shape=Subtype,
group=Subtype,
linetype=Subtype))+
geom_point(size=2)+
geom_line()+
ylab("Porduction performance on all verbs")+
xlab("Treatment phase") +
scale_linetype_manual(
values=c("longdash","solid", "dashed", "dotted")
)