The nassi program developed at the Jülich Supercomputing Centre (JSC) at the Forschungszentrum Jülich is a tool to generate Nassi-Shneiderman diagrams under Unix/X11. nassi is designed primarily for the creation of documentation, but may as well be used for source code analysis/browsing.
C and PASCAL programs and pseudo code that follows the conventions of either of these languages can be transformed into a graphical representation of the program flow. For representation and postprocessing nassi provides a convenient interface with which single diagrams can be selected, drawn on the screen, exported in several output formats and printed.
A graphics editor allows layout changes of the whole diagram or of single statements or control structures. Statements and whole structures can be hidden or shifted as a block to a separate diagram. Such changes to the layout and structure of diagrams can be inserted into the source code via special comments and are then available for further treatment of the source code.
For output purposes, nassi also provides the option of generating source data of the Tgif and Xfig graphics editors in addition to screen output, Encapsulated PostScript graphics and printable PostScript files. This makes it possible to also change and extend diagrams far beyond the functionality of the built-in graphics editor. For the rapid generation of diagrams nassi provides a batch option which generates diagrams in the desired output format.
For details see the documentation or take a look at some screen shots in the tutorial.