[graphviz-interest] forcing distinct/more ranks?

I am (one of the many) using GraphViz to make a family tree diagram.

In a bid to get something sensible and usable, I am being
slightly unconventional, and allowing graphviz more flexiblility than usual.
Instead of (classically) setting "rank=same" on husbands and wives (nodes) in a marriage,
and setting "rank=same" on their offspring sets (which we might call "generation centric"), I am
setting "rank=same" on person (nodes) of the same birth
year (and, in fact),marriage (nodes) of the same year).

In this way, I not only spread the nodes about a bit, but I also endeavour
to make the diagram more time-line like.

However, GraphViz is (cleverly) spotting that it doesn't NEED all the ranks
(implied by year) that I'm offering it, so that persons with different birth
dates appear on the same rank (Y position).

GraphViz is behaving exactly as I'm telling it.

Question - how can I force it (or persuade it...)
to put nodes on separate ranks, even if it doesn't need to?

    BugBear
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