onCompute method

@override void onCompute(Size size)

获取画布宽度和高度 计算当前屏内相对转换后的X轴位置 根据X轴位置，计算屏内起始和结束展示数据的索引 计算最大值和最小值

Implementation

@override
void onCompute(Size size) {
  super.onCompute(size);
  double diff =
      math.max((maxValue - lastClose!).abs(), (minValue - lastClose!).abs()) *
          1.5;
  if (diff < 1) {
    diff = 1;
  }
  maxValue = lastClose! + diff;
  minValue = lastClose! - diff;
  _radio = contentHeight / (maxValue - minValue);
}