type: clusterMode, default:
subgraph whose name begins with
cluster is given special treatment.
The subgraph is laid out separately, and then integrated as a unit into
its parent graph, with a bounding rectangle drawn about it.
If the cluster has a
label parameter, this label
is displayed within the rectangle.
Note also that there can be clusters within clusters.
clusterrank=none appear to
be identical, both turning off the special cluster processing.
Note: dot only.