packages/victory-stack/src/helper-methods.js
function fillData(props, datasets) {
  const { fillInMissingData } = props;
  const xMap = datasets.reduce((prev, dataset) => {
    dataset.forEach((datum) => {
      prev[datum._x instanceof Date ? datum._x.getTime() : datum._x] = true;
    });
    return prev;
  }, {});
  const xKeys = keys(xMap).map((k) => +k);
  const xArr = orderBy(xKeys);

  return datasets.map((dataset) => {
    let indexOffset = 0;
    const isDate = dataset[0] && dataset[0]._x instanceof Date;
    const filledInData = xArr.map((x, index) => {
      x = +x;
      const datum = dataset[index - indexOffset];

      if (datum) {
        const x1 = isDate ? datum._x.getTime() : datum._x;
        if (x1 === x) {
          return datum;
        } else {
          indexOffset++;
          const y = fillInMissingData ? 0 : null;
          x = isDate ? new Date(x) : x;
          return { x, y, _x: x, _y: y };
        }
      } else {
        const y = fillInMissingData ? 0 : null;
        x = isDate ? new Date(x) : x;
        return { x, y, _x: x, _y: y };
      }
    });

    return filledInData;
  });
}
function fillData(props, datasets) {
  const { fillInMissingData } = props;
  const xMap = datasets.reduce((prev, dataset) => {
    dataset.forEach((datum) => {
      prev[datum._x instanceof Date ? datum._x.getTime() : datum._x] = true;
    });
    return prev;
  }, {});
  const xKeys = keys(xMap).map((k) => +k);
  const xArr = orderBy(xKeys);

  return datasets.map((dataset) => {
    let indexOffset = 0;
    const isDate = dataset[0] && dataset[0]._x instanceof Date;
    const filledInData = xArr.map((x, index) => {
      x = +x;
      const datum = dataset[index - indexOffset];

      if (datum) {
        const x1 = isDate ? datum._x.getTime() : datum._x;
        if (x1 === x) {
          return datum;
        } else {
          indexOffset++;
          const y = fillInMissingData ? 0 : null;
          x = isDate ? new Date(x) : x;
          return { x, y, _x: x, _y: y };
        }
      } else {
        const y = fillInMissingData ? 0 : null;
        x = isDate ? new Date(x) : x;
        return { x, y, _x: x, _y: y };
      }
    });

    return filledInData;
  });
}
Assumes data in `datasets` is sorted by `Data.getData`.