Bubble up chart events

[silvester-pari]

One example use case when using the chart in eodash is to listen to click or selection events in order to react to the selection being done.
We would use this to react to the datetime property if available on the selected point, in order to set the corresponding time in the the eodash client.
So a helper to allow to listen to "point" selected, or click event of the chart would be great.

*edit:
In the AgriApp this functionality is also needed to set the date of the app and load corresponding satellite images

[StefanBrand]

Some related info: https://stackoverflow.com/questions/57707494/whats-the-proper-way-to-implement-a-custom-click-handler-in-vega-lite