An exception is Pictorial Janus [6, 10] in which the syntax is defined in terms of the topological relations between picture elements.
A more fundamental problem that Pictorial Janus shares with most visual programming systems is that formal diagrams are used to encode programs and many people find formal diagrams difficult to understand and construct.
Pictorial Janus, for example, is relatively difficult to learn but is very flexible.
Someone familiar with Pictorial Janus can draw on paper the equivalent program in the same amount of time.
If one compares equivalent textual and Pictorial Janus programs constrained to be easy to read then they use about the same area on the screen.
The slot machine, Pictorial Janus and ToonTalk all provide a means of seeing the state of an entire computation in the same visual terms or syntax as the programs being executed.
Except for Pictorial Janus, this isn't a practical problem since in other visual programming systems and in ToonTalk, programs can only be constructed using a specialized piece of software.
A postdoc (Markus Fromherz) is using Pictorial Janus to model the behavior of paper paths in copiers.
My research on Pictorial Janus convinced me that encoding the desired behaviors as static diagrams was a good step in the right direction, but not a large enough one.