[Feature] bar 系列支持堆叠柱整体圆角

[zamhown]

类似于如下效果：

可以用以下 bar.style.cornerRadius 回调模拟，虽然在回调里不涉及具体数据，但是仍然和数据关联性较大，不好配 + 容易出 bug。可以考虑在系列中从图元层面添加圆角。

const spec = {
  type: 'bar',
  data: [
    {
      id: 'barData',
      values: [
        {
          State: 'WY',
          Age: 'Under 5 Years',
          Population: 25635
        },
        {
          State: 'WY',
          Age: '5 to 13 Years',
          Population: 1890
        },
        {
          State: 'WY',
          Age: '14 to 17 Years',
          Population: 9314
        },
        {
          State: 'DC',
          Age: 'Under 5 Years',
          Population: 30352
        },
        {
          State: 'DC',
          Age: '5 to 13 Years',
          Population: 20439
        },
        {
          State: 'DC',
          Age: '14 to 17 Years',
          Population: 10225
        },
        {
          State: 'VT',
          Age: 'Under 5 Years',
          Population: 38253
        },
        {
          State: 'VT',
          Age: '5 to 13 Years',
          Population: 42538
        },
        {
          State: 'VT',
          Age: '14 to 17 Years',
          Population: 15757
        },
        {
          State: 'ND',
          Age: 'Under 5 Years',
          Population: 51896
        },
        {
          State: 'ND',
          Age: '5 to 13 Years',
          Population: 67358
        },
        {
          State: 'ND',
          Age: '14 to 17 Years',
          Population: 18794
        },
        {
          State: 'AK',
          Age: 'Under 5 Years',
          Population: 72083
        },
        {
          State: 'AK',
          Age: '5 to 13 Years',
          Population: 85640
        },
        {
          State: 'AK',
          Age: '14 to 17 Years',
          Population: 22153
        }
      ]
    }
  ],
  xField: 'State',
  yField: 'Population',
  seriesField: 'Age',
  stack: true,
  legends: {
    visible: true
  },
  bar: {
    style: {
      cornerRadius: (data) => {
        const series = vchart.getChart().getAllSeries()[0];
        const statisticsData = series.getViewDataStatistics().latestData;
        const seriesField = series.getSeriesField();
        const seriesFieldValues = statisticsData[seriesField].values;
        if (data[seriesField] === seriesFieldValues[seriesFieldValues.length - 1]) {
          return [20, 20, 0, 0]
        }
        return 0
      }
    }
  }
};
const vchart = new VChart(spec, { dom: CONTAINER_ID });
vchart.renderSync();
// Just for the convenience of console debugging, DO NOT COPY!
window['vchart'] = vchart;