Fit functions for stacked line and area charts

[nickofthyme]

Is your feature request related to a problem? Please describe.
Lens would like to have the ability to create stacked area/line charts with null y1 data.

#388

Describe the solution you'd like
The solution would be to use the fit function logic implemented in the non-stacked charts.

Describe alternatives you've considered
Currently, the workaround is to fill the null data in kibana or have a strange-looking chart

Additional context
To do this for stacked charts requires each successive series to know the values of the series below at that point. If series a has no explicit value at x = 1 but has an interpolated value of 2 based on the bounding values, then we need to first find the missing value to enable stacking of series b at x=1.

Further complications come into play when adding a curve type to the series, where the calculated value is not exact.

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

[markov00]

🎉 This issue has been resolved in version 21.0.0 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

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