Allow independent series fitting on x axes

[nickofthyme]

Is your feature request related to a problem? Please describe.
Yes, the issue is that when using any fit type on a series when there are multiple series specs declared, this causes the fit logic to fill the gaps. This treats all series as if they represent the same data, more specifically it assumes all x values are shared across all series effectively coupling all series to each other.

See https://codesandbox.io/s/nostalgic-fast-q5q88?file=/src/App.tsx for an example like this...

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Another case that could apply to this fitting logic is if the x values are nearly matching but shifted by some infinitesimal amount. If both series are using some fit type, they will both render as if they have discontinuous data.

See https://codesandbox.io/s/zen-shamir-pe4ho?file=/src/App.tsx for an example of this...

In this image the threshold is 1 minute ahead of the actual data timestamp.

Describe the solution you'd like
Allow different series specs to be independent in terms of fitting logic. This would not affect the domain extents just how the fitting is applied.

Describe alternatives you've considered

• Don't use series fitting at all
• Fill values to match each series domain where needed.