Read from network attributes

[briatte]

Some arguments for nodes, labels and possibly edges might be read from the network object attributes, as in this example that reads the mode.

Arguments that could be read from attributes:

• weight.method (e.g. reading weighted centrality from the nodes)
• node.group, node.alpha, node.color (would avoid problems when passing color scale values)
• segment.alpha, segment.color and segment.size (using get.edge.attribute)
• label.nodes and new arguments like label.color and label.alpha (yet to be added)

The issue is: how to distinguish network attribute names from colors?

[briatte]

Reading vertex attributes (including the mode) is now implemented in ggnet2.