Tooltip: Align existing functionality across chart types

[nikolasrieble]

Context: #3380

The Tooltip currently shows different behaviour between ComposedChart / ScatterChart.
For a ScatterChart, the Tooltip shows exactly one point, independent of other points that might have the same xAxis value.

This feature is something that many users requested for the ComposedChart and due to the lack if workaround were needed.
(TODO: Find references / issues / workaround links).

On the other hand one might equally want to show in a ScatterChart a Tooltip in the manner of the ComposedChart, i.e. a single Tooltip for all entries with the same xAxis value.

I suggest to extend the Tooltip with a new field such as mode: 'single' | 'xAxis'.

I am confident this has high user value, it would further simplify and align our API across Charts.

[nikolasrieble]

@andrewangelle You might like this problem. Either way curious to hear your perspective.