code: function downsample(data, maxPoints, startingIndex = 0) {
    // ensures that the downampling of data while zooming looks good.
    const dataLength = getLength(data);
    if (dataLength > maxPoints) {
      // limit k to powers of 2, e.g. 64, 128, 256
      // so that the same points will be chosen reliably, reducing flicker on zoom
      const k = Math.pow(2, Math.ceil(Math.log2(dataLength / maxPoints)));
      return data.filter(
        // ensure modulo is always calculated from same reference: i + startingIndex
        (d, i) => (i + startingIndex) % k === 0
      );
    }
    return data;
  }
docs: Reduces the size of a data array, such that it is <= maxPoints.
  @param {Array} data: an array of data; must be sorted
  @param {Number} maxPoints: maximum number of data points to return
  @param {Number} startingIndex: the index of the data[0] *in the entire dataset*; this function
  assumes `data` param is a subset of larger dataset that has been zoommed
  @returns {Array} an array of data, a subset of data param