如何使用getData获取webdatarock上的列总数？

// 创建webdatarock实例
var pivot = new WebDataRocks({
  container: "#pivotContainer",
  toolbar: true,
  report: {
    dataSource: {
      data: [
        { "Product": "A", "Category": "X", "Sales": 100 },
        { "Product": "B", "Category": "Y", "Sales": 200 },
        { "Product": "C", "Category": "Z", "Sales": 300 }
      ]
    },
    slice: {
      rows: [{ uniqueName: "Product" }],
      columns: [{ uniqueName: "Category" }],
      measures: [{ uniqueName: "Sales", aggregation: "sum" }]
    }
  }
});

// 在数据加载完成后获取列总数
pivot.getData(function() {
  var columnCount = pivot.getNumberOfColumns();
  console.log("列总数：" + columnCount);
});