createGridlineStyle method
Creates LineStyle for axis gridlines from spec.
Fill missing value(s) with default.
Implementation
@override
LineStyle createGridlineStyle(
    GraphicsFactory graphicsFactory, LineStyleSpec? spec) {
  return graphicsFactory.createLinePaint()
    ..color = spec?.color ?? MaterialPalette.gray.shade300
    ..dashPattern = spec?.dashPattern
    ..strokeWidth = spec?.thickness ?? 1;
}