I am learning analysis of longitudinal data from the  book 
Longitudinal data Data Analysis for the Behavioral Science using R by Jeffrey 
Long. 
On page 123, 

myX <- scale_x_continuous(breaks = 5:8, name = "Grade") 
myY <- scale_y_continuous(name = " Reading Score ") 

g1 <- ggplot(data=MPLS.LS, aes(x=grade, y=read)) + geom_point(shape =1) + myX 
+myY 
g2 <- g1 + stat_summary(fun.y = mean, geom = "line", lwd = 1.5, linetype = 5) 
print(g2) 

It refused to draw the curve of the means, with an error message: 
geom_path: Each group consist of only one observation. Do you need to adjust 
the group aesthetic? 

But it doesn't give the error message if I used instead 
g3 <- g1 + stat_summary(fun.y = mean, geom = "point", size=3, shape = 19) 
print(g3) 

Any advice would be  greatly appreciated. 

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