getMeanAndStddev(dataColumnName) {
    if (this.means == null || this.stddevs == null) {
      throw new Error('means and stddevs have not been calculated yet.');
    }

    const index = this.getDataColumnNames().indexOf(dataColumnName);
    if (index === -1) {
      throw new Error(`Invalid data column name: ${dataColumnName}`);
    }
    return {
      mean: this.means[index], stddev: this.stddevs[index]
    }
  }
Get the mean and standard deviation of a data column.