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To enable the job diagram viewer, please install install graphviz dot command. To check, wether it is available, open a shell and type type dot -?
Hint: If dot is not available JOE writes the value false to JavaSoft/Prefs/com/sos/joe/globals/options/properties/com.sos.joe.globals.options./Options/editor.jobchain.digagram.show
Graphviz can be downloaded from here: http://www.graphviz.org/Download..php
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