Allow users to configure fitting/filling function for missing/null buckets

[adrien-cahoreau]

Is your feature request related to a problem? Please describe.

Missing buckets are handled a bit differently depending on the chart, e.g. :

• In line charts, in case of missing buckets between valid buckets, the line is traced directly between valid buckets, "hiding" the missing buckets.
• In TSVB, in case of missing buckets, a 0 value is displayed, so the line follows these 0 values, making clear there are missing buckets.

Describe the solution you'd like

Since the way a null value should be handled really depends on the semantics of the data, we cannot make a meaningful decision for the user. We should rather give the user the possibility to change a "fitting function", that will calculate what values should be "fitted" in for null values. Possible values could be (the examples, show what that fitting function would turn the input series [2, null, null, 8] into):

Name	Description	Examples
None	Don't draw that value on the graph	[2, null null, 8]
Carry	Use the last non null value before that	[2, 2, 2, 8]
Nearest	Use the closest value (either before or after) that was non null	[2, 2, 8, 8]
Lookahead	Use the next non null value after that (opposite of Carry)	[2, 8, 8, 8]
Average	Use the average of the last and next non null value	[2, 5, 5, 8]
Linear	Linear interpolate between closest values	[2, 4, 6, 8]
Zero	Replace values with 0	[2, 0, 0, 8]
Explicit	Specify an explicit value (x), that should be used instead	[2, x, x, 8]

This fitting settings would behave similar to the Timeline .fit function, but should work for all charts.

Describe alternatives you've considered

Kibana/Lens offers a fitting function for missing buckets since Kibana 7.9.0.

Additional context

Equivalent to elastic/kibana#17717

[kavilla]

Thanks for opening @adrien-cahoreau,

Tagging @opensearch-project/opensearch-ux.

[ahopp]

@adrien-cahoreau thanks for opening! This would be great to add.