Robtandy/better graphviz plans

[robtandy]

As a PR description, this query

SELECT COUNT(o.o_orderkey) AS order_count, c.c_name
FROM orders AS o
JOIN customer AS c
ON c.c_custkey = o.o_custkey
GROUP BY c.c_name
ORDER BY order_count desc;

Now produces this graphviz output with partitions = 4, max_partitions_per_task=2