You are watching the raw data. To visualize it: https://openwsn-berkeley.github.io/mercator/.

Mercator is a collection of connectivity datasets gathered on the IoT-LAB sites. These datasets are:

• dense in time, meaning the connectivity is continuously assessed over a long period of time; it allows one to see variation of connectivity over time.
• dense in space, meaning the connectivity is assessed over hundreds of measurements points; it allows one to see how connectivity is affected by the location of transmitter and receivers.
• dense in frequency, meaning the connectivity is assessed for each of the 16 IEEE802.15.4 frequencies; it allows one to see how connectivity is affected by the communication frequency.

Want to learn more? See the wiki.