Cleaned up a few things regarding multiple data sources as used in network viz.
Added an 'init_chart' function which called once before the first call to 'update'. Is a NOOP here, but now used in network2 to set up the layouter.
Added support for automatic node placement using the d3.force layout. Simply map the x and y properties for nodes to 'auto'. See 'topo_discovery' for an example.
Added new example which uses the network viz with force layout.