Ya'll might want to check out jpgraph (www.jpgraph.org). I know its for PHP4, but it does some really kewl stuff (like built-ins for Gannt Chart generation)
If you mount the latest copy of GForge (www.gforge.org), you can see it in action, creating Gannt Charts on the fly (http://gforge.org/pm/task.php?group_id=5&group_project_id=2&func=ganttpage).
As for Caroline's requirement, have you tried checking out Dia? (http://www.gnome.org/projects/dia/)
