Parsing of visual structures like diagrams and graphs is more complicated than parsing strings. This is so because visual structures are inherently more complex than strings, because visual grammars are more difficult to write than string grammars, and because the algorithms for parsing visual structures are usually more complicated than for parsing strings. The developer of a visual parser, therefore, needs more tool support than a developer of a string parser. In fact, developing and debugging a visual parser without proper visualization of the parsing process is very challenging. This paper describes a visualization approach that arose from this need. Its main focus is on the interaction of the developer with the visualization tool in order to explore the execution process of the parser. It has evolved from experiences with developing and debugging parsers by applying different visual parsing methods. In order to better describe it we introduce a concrete example.