Improve web app visualization

- [ ] Plumb along and display zones for origins/destinations too?
- [ ] For zone-based origins/destinations, also visualize the `od.csv` outputs
- [ ] Scale O/D circles based on count, and include histograms for them too?
- [ ] Try rank/ordering for line width (1st most important edge, 2nd...)
- [ ] Auto-set the max for line width value, by plumbing along the max count in the output metadata