With this option the writer to use to create graph data from the evaluation
results may be specified. All writers use the column definitions
defined by the option(s) [@o]--column-definition.

Currently the following graph writers are implemented:

* The flat graph writer prints the results in a two-dimensional table
  to stdout. The first line consists of the headlines, the remaining
  lines contain the graph data. This writer has no additional arguments.
  
* The family graph writer prints a family of graphs as two-dimensional
  tables to stdout. Graphs are collected according to the value of a given column.
  This writer requires exactly one numeric parameters which defines the column to
  use as collection criteria (family column).

If these writers are not sufficient a user written external graph writer
may be called which can access the SimTree internal data structure
describing the results of an evaluation. The interface which an external
graph writer must implement, some internal SimTree classes and helper
methods are provided in an example package for writing an external
graph writer.

When this option is not given, the flat graph writer is used.
