Feature/single data area chart

[leechoohyoung]

New Pull Request

Summary

• Added a new option: plotOptions.area.forceSingleDataAsArea.
• When this option is enabled, area charts with only a single data point will render the area (filled region) instead of just a marker.
• Previously, area charts with a single data point would only display a marker. With forceSingleDataAsArea: true, the chart will visually fill the area beneath the single point, maintaining the area chart style.

Motivation and Context

• This feature was added in response to my requesting that area charts with a single data point still display as an area, not just a marker.
• It is useful for cases where data is sparse or changes in real-time, and a consistent area chart appearance is desired.
• This change is backward compatible; the default value is false, so existing behavior is preserved unless the option is explicitly enabled.

Fixes

Fixes # (issue)
※ Please fill in the issue number if applicable.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• My branch is up to date with any changes from the main branch