graphs: make graphs with lots of packages nicer to look at

Some magic numbers obtained with trial-and-error and successive
iterations, to eventually get a nice graph.

[Thomas: remove excessive spaces in expressions.]

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>