How to understand graphviz dot edge and node behavior?

Question

Coming from this graph:

digraph G {

a0 -> a1 -> a2;
x0 -> x1;
b0 -> b1 -> b2;

}

which is giving this graphical representation:

a 0    x 0    b 0
 |      |      |
a 1    x 1    b 1
 |             |
a 2           b 2

I'd like to connect a2 to x1 and x1 to b2.

So I tried:

digraph G {

a0 -> a1 -> a2;
x0 -> x1;
b0 -> b1 -> b2;

a2 -> x1 [constraint=false];
x1 -> b2 [constraint=false];

}

My expectation would be this:

a 0    x 0    b 0
 |      |      |
a 1    x 1    b 1
 |  /       \  |
a 2           b 2

But unfortunately it is rendered like this and I have no clue why.

a 0    b 0    x 0
 |      |      |
 |      |      |
 |      |      |
a 1    b 1    x 1
 |      | _ / / 
 |      +    / 
 |  _/   |  /  
a 2    b 2

How can I gain a better understanding of why is that and help me to keep x0 -> x1 in the middle of the graph?

Answer 1

Adding {rank=same; a0 -> x0 -> b0 [style=invis];} to your graph gives the desired result - this forces the top nodes to be in a specific order, but hides the edges added to do this. I have absolutely no idea why this should be necessary, however.