connect lines in R program/ggplot2 function?

Question

ggplot(sumData, aes(x=supplement, y=MeanGain, colour=diet))+

geom_point()+

geom_errorbar(aes(ymax= MeanGain+ St.erGain,

ymin= MeanGain-St.erGain), width= 0.1)+

ylab("Mean Swiss cow weight gain")+

ggtitle("MEAN AGAINST SUPPLEMENT")+

geom_line

Where is my mistake? why the lines appear only on the small boxes and not in the graph?

Update:

geom_line () 

I have included the brackets that's not the mistake, is not working either