Let the user select events for similarity analysis

The user has to select multiple events (at least two), while one of those selected events will be marked as the reference event. The selection shall be done via clicking on the event bars in the event history chart.

Ideas for selecting the reference event:

1. each selected event/day ends up in a list of events. the user can then select the reference in this list, e.g. via checkbox and remove selections, etc.
2. keep everything in the chart and simply click on an already selected event again to mark this as the reference. A right, double or third click might remove the selection.

While the user selects events as single days, it's at the discretion of server-side logic module to define the actual timerange of the individual events. Those might end up being several days or even weeks. This has to be taken into account, when the modules results are received.