Assuming there is a plain text with description of the workflow (Just plain English in some predefined format).
Are there any tools (better online) to visualize the flow based on a plain text?
What for: to store the description of the workflow in a source control system and be able to quickly remember/understand that.
Honestly I find the idea appealing but doubt it could be practical. You mention "plain text" but not sure if this means source code or a natural language description.
If it is source code maybe you can find something that automatically generates flowcharts from a specific language (you could have better luck if you stated what the language is).
If you are trying to extract a flowchart from a high-level English (or whatever) language descriptions I think it will prove quite a challenge.
If I had to do this, I'd go with Perl starting by http://search.cpan.org/~xern/Text-Flowchart-Script-0.02/Script.pm for the "rendering" while reading a bit on NLP for the parsing (ex.: http://www.perlmonks.org/?node=41281).