I'm trying to visualize the history of my source-code repository using Graphviz in the following fashion (top-to-bottom or left-to-right doesn't matter):

m1 -> m2 ----> m3 -----> m4 -> m5
    \                    ^
     \-> b1 -> b2 -> b3 -/

Given this dot file:

digraph git {
    subgraph master {
        m1 -> m2 -> m3 -> m4 -> m5
    }
    subgraph branch {
        m2 -> b1 // branch from master
        b1 -> b2 -> b3
        b3 -> m4 // merge into master
    }
}

Graphviz diagram

what attribute should I set to have all the nodes of a sub-graph (that belongs to the same branch) to be placed on a straight-line?

graphviz
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