Gnuplot version constraint

[rhymecat]

The use of named blocks ($data << EOD) which is used in src/report/NodeDensityReport.java requires Gnuplot version 5 (see FAQ 4.8).

I guess that should be documented somewhere.

[tk721]

The named blocks are used to have the data and the code in one file. The workaround would be to output two files, one for data and one for code, but currently the ONE cleanly supports only outputting one file per report module.