During Zurich spring festival thousands of people parade through the Old Town and at 6pm they set a on fire to mark the end of winter. You can read more about this tradition here.
The bars on the map display the pedestrian counts at various locations in Zurich on the 8th of April 2019, during the spring festival. Each counter shows the number of pedestrians every 15 minutes, from 1 person to 400 persons .
Zurich Spring Festival parade passes this point.