Zero-based graph_id numbering
The current graph_id-numbering starts from 1, but e.g. the simulation output starts from 0. This should be consistent (and starting from 0). May require a bit of modification to tests etc.
The current graph_id-numbering starts from 1, but e.g. the simulation output starts from 0. This should be consistent (and starting from 0). May require a bit of modification to tests etc.