Let's say I have six dots:
 x_11, x_12, x_13, x_21, x_22, x_23.
I want to draw lines that connect x_11 with x_12 and x_13, but not a line between x_12 and x_13. 
After that, I want to do the same thing to x_21, x_22, and x_33. 
That is, I want to connect x_21 with x_22 and x_23, but not x_22 and x_23. 
I tried the group method (adding the group attribute in aes) but it connects all three dots.
Is there a way to conveniently solve the problem?
 
    