customizeSegment method
Changes the series color, border color, and border width.
Implementation
@override
void customizeSegment(ChartSegment segment) {
  final BubbleSegment<T, D> bubbleSegment = segment as BubbleSegment<T, D>;
  updateSegmentColor(bubbleSegment, borderColor, borderWidth);
  updateSegmentGradient(
    bubbleSegment,
    gradientBounds: bubbleSegment.segmentRect,
    gradient: gradient,
    borderGradient: borderGradient,
  );
}