customizeSegment method
Changes the series color, border color, and border width.
Implementation
@override
void customizeSegment(ChartSegment segment) {
  final StackedBarSegment<T, D> stackedBarSegment =
      segment as StackedBarSegment<T, D>;
  updateSegmentTrackerStyle(
      stackedBarSegment, trackColor, trackBorderColor, trackBorderWidth);
  updateSegmentColor(stackedBarSegment, borderColor, borderWidth);
  updateSegmentGradient(stackedBarSegment,
      gradientBounds: stackedBarSegment.segmentRect?.outerRect,
      gradient: gradient,
      borderGradient: borderGradient);
}