A data provider key that can be used to register a
DataProvider for each Node in the graph that yields a Comparable object
via the DataProvider.get(Object) method.
The values can be used by the various tree layouter implementations to sort the children
of each node in the tree and use the order to influence the layout.
To achieve this use this comparator and assign it to the layouter implementations
comparator property.
Object is actually expected to be Edge.
Compares values returned by DataProvider.get(Object) for
the target nodes of the edges.
null values or instances that do not implement the Comparable
interface will be treated as (equal) smaller values.