algorithmic modeling for Rhino
We use SDNA plugin in QGIS for measuring the connectivity of urban networks and then import the results into Grasshopper for visualisation. That doesn't have much to do with "visual connection", rather spatial configuration of links.
If you're interested in playing with isovists then DeCodingSpaces Toolbox should help. I haven't used it myself yet, but there are some video tutorials avaialable online.