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Just guessing ...
Maybe plotutils cannot be found because its path is not visible ?
What happens if you run the example from
 
_http://ruby-gsl.sourceforge.net/rnd3.html_ 
(http://ruby-gsl.sourceforge.net/rnd3.html)  .
 
If that works out ok, you could execute the graph command
as an external command, eg. by including it in backticks or
by calling the system command ...
 
Best regards,
 
Axel 

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