Improve visualization

[oerich]

The visualizations are good for exploring their power, but not so much for using them. They could be more convincing. Here is how to do that:

• Use trajectories to give an overview of hotspots in a list of stories (#59)
• Couple detailed view of trajectories with sna visualization (#60)
• Offer a more systematic representation for trajectories, based on the new pattern system (#65)