The output of the step is an abstract representation of a solution to the problem. Once there is a representation of the problem in the input language of the step, it is important not to modify the output from the step or the chain is broken. A graphical modeling language represents the problem using both graphical and textual descriptions. There are many scenarios in which a pictorial representation is easier to comprehend than a textural description. For example, in describing a collision between three vehicles, people will often draw a pictorial representation of the problem space rather than trying to describe the problem using only words.
People use different high-level languages.
In the same way, the symbols used for the pictorial representation may vary to meet the needs of the problem space. For example, some areas may use Shlaer-Mellor notation, while others might use Rumbaugh notation. Thus, it is possible to have multiple graphical modeling languages. Some people haven't seen the need for graphical models. I believe that at this point, if not before, the need for graphical modeling tools will become increasingly apparent.