Reports

Right, this is so straightforward it is embarrassing. The key was to group by the two map keys that I wanted to keep, and there's a handy groupCount() to make it even easier. Like this:

g.V().hasLabel('city').
    out('hasParking').
    out('hasSlot').
    inE('parkedAt').has('time', P.lte(2)).has('time', P.gte(1)).
    project('city', 'parking', 'vehicle').
    by(inV().in('hasSlot').in('hasParking').values('name')).
    by(inV().in('hasSlot').values('name')).
    by(outV().values('name')).
    dedup().
    groupCount().
    by(select('city', 'parking')).
    unfold()

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