Do not force stacking on single histogram series

[nickofthyme]

Describe the issue

Whenever a user creates a bar series and enables the enableHistogramMode flag, the series is treated as stacked.

This unexpectedly disabled the domain fitting due to the filling of dataseries values for stacked series.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://codesandbox.io/s/empty-pond-lfn4t2
2. Toggle enableHistogramMode option
3. Notice how the domain fit is applied in each case

Expected behaviour
Domain fit should not be impacted by whether the enableHistogramMode flag is enabled or not.

Version (please complete the following information):

• Elastic Charts: v21.0.0

Additional context

This issues was introduced in #751 by treating any bar series with enableHistogramMode set to true as stacked. This was done to prevent clustered bar charts in a histogram.

The solution here would be to only force the stacking behavior for histogram bar charts when there is more than one bar series.

Kibana Cross Issues

• [Agg-based] Custom data bound limited for all xy chart types other than lines kibana#170316

Checklist

• The proper chart type label has been added (e.g. :xy, :partition)
• The proper feature labels have been added (e.g. :interactions, :axis)
• Every related Kibana issue is listed under Kibana Cross Issues list and the kibana cross issue tag is applied