Adding cause-effect graph (#1674)

Hi Dusan,

I was wondering if you plan to add cause-effect graphs as well in the future. These are also very simply graphs, but are very useful for creating/visualizing decision tables.

Here's a pdf about it:

http://www.westfallteam.com/Papers/Cause_and_Effect_Graphing.pdf

Perhaps you could also add a feature which would allow to convert the graph to a table or the other way around (table to graph), but that of course is just another idea.

Thanks for considering.