Is your feature request related to a problem? Please describe.
Add a band area chart like in the following screenshot.
TSVB currently has this function when using percentile aggregation in band mode.

Describe the solution you'd like
The area chart specification can be extended to add a y0 accessor to specify the minimum value of the area chart if not zero. This will be used to compute the band.

Describe alternatives you've considered
n/a

Additional context
n/a

Kibana Cross Issues
n/a

Checklist

• this request is checked against already exist requests
• every related Kibana issue is listed under Kibana Cross Issues list
• kibana cross issue tag is associated to the issue if any kibana cross issue is present