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I'm not positive what you mean by "producing diagrams from text", but if you mean a tool where diagrams are specified by a text file, Graphviz is good. If you mean something that literally converts ascii art like
+--------+ +-------+ +-------+
| | --+ ditaa +--> | |
| Text | +-------+ |diagram|
|Document| |!magic!| | |
| {d}| | | | |
+---+----+ +-------+ +-------+
: ^
| Lots of work |
+-------------------------+
to a graphic:

You can try ditaa (that ascii art is from their website, so it's a good example of the input format it expects)