pack has an integral value, this is used as the size,
in points,of a margin around each part; otherwise, a default
8 is used.
If pack is interpreted as
false, the entire graph is laid out together.
The granularity and method of packing is influenced by the
For layouts which always do packing, such as
attribute is just used to set the margin.
pack is treated as true if the value of pack is
or a non-negative integer.