df1<-data.frame(x=c(3,4,5),y=c(15,20,25),grp=c(1,2,2))
p<-ggplot(df1,aes(x,y))
p<-p+ geom_point(aes(colour=factor(grp),size=4))
p

df2<-data.frame(x=c(3.5,4.5,5.5),y=c(15.5,20.5,25.5))
p<-p + geom_path(data=df2,aes(x=x,y=y))
p

df1$grp = factor(df1$grp)
colnames(df1)[3] = "Group"

p = ggplot(df1,aes(x,y))
p = p+ geom_point(aes(colour=Group), size=4)

p =  p + scale_colour_discrete(name="Group")

ggplot(df1,aes(x,y)) + geom_point(aes(colour = factor(grp)), size=4) +
scale_colour_discrete(name = "Grp")