1.3.1 legend related scales in viz + title in meta

[paulgirard]

This 1.3.1 proposal aims at adding to GEXF data to document graph drawing with legend and title. GEXF viewer tools (such as https://gitlab.com/ouestware/retina/) can't so far indicate what is the logic behind the visual aspects without reverse engineering the viz parameters/node attributes sets with some heuristic magic. A good graph drawing often also starts with a good title on top of description.

Therefore this proposals has two main parts:

• add a title element in the GEXF meta
• extend the viz module to add ways to describe how the viz parameters were calculated from node/edge attributes. It adds ways to store the ranking/partition parameters and layout settings used in Gephi or in other GEXF producers. It has primarily a documentation objective but the current specs looks complete enough to allow drawing tools to not only draw a legend but also recompute the viz parameters from attributes.

To get a more precise idea of the proposal see:

• the new 1.3.1 primer section about scales: gexf-131-primer-legend.pdf
• the extended viz rnc file: https://github.com/gephi/gexf/blob/legend/specs/1.3.1/_viz.rnc#L82-L158

ping @duncdrum and @gvegayon for comments.