Show density chart as background for slider

[peterdesmet]

Show the average density over all 5 radars as a timeseries line chart. The chart should be used as the background for the time slider, giving some context to the user which timeframes are interesting to look at (those with high density). That would be a good alternative for #58.

Best to implement this in C3.js

[peterdesmet]

Implemented a first version in /experiments. Draw for hoover line is still too slow (uses chart.xgrids):

[hvangasteren]

Very cool already guys, but keep in mind that when expanding to larger areas this is not possible anymore. Shouldn't we start next week 👍