Refactor: Remove global offset cache in LineChartPainter (per-instance / safe)

[banoth-ravinder]

Summary

Refactor plot-point offset handling in LineChartPainter so it is safe and does not rely on global state.

Problem

• LineChartPainter used a static Map<LineChartBarData, List<Offset>> _cachedOffsets to cache pixel positions for line series.
• This caused:
  • Shared state across all chart instances.
  • Unbounded growth: the map was never cleared, so memory could grow over time.
  • Stale/cross-instance reuse: new LineChartBarData instances (e.g. after refresh) could be confused with old keys (Equatable-based), or different charts could affect each other.
• The cache was also never populated (no write after computing), so it was dead code and still unsafe.

Solution

• Remove the static _cachedOffsets map entirely.
• Compute plot points in createPlotPoints(rect, lineBarData) on each paint; no global or static cache.
• Behavior is unchanged (we were already computing every time); the unsafe/dead cache is removed.

Files needs to be changed

• lib/src/line_chart/line_chart_painter.dart

Checklist

• Remove static _cachedOffsets.
• Make createPlotPoints stateless (compute and return only).
• Update doc comment for createPlotPoints.
• No new linter issues.