Show appropriate date when date is part of grouped data on chart.

[onesimus-wiafe]

This PR addresses issue #2566. When the data is grouped and a date is part of the grouped data, the date values are then represented in epoch format. I couldn't figure out the exact source of the problem, and this solution addresses the problem. I included a function in the filter.ts file since all the plugins affected by this issue import the functions in filter.ts. I refine the date type values before the functions in filter.ts run. The tests included confirm this solution too.

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