React: Can't perform a React state update on an unmounted component

you are not returning data from addQuarterStringsToArr function that why here setData(addQuarterStringsToArr(data)) going to be undefined.

just return dataset in addQuarterStringsToArr function like this:

// helper function to add corresponding text for the quarter and year as a string
export default function addQuarterStringsToArr(dataset) {
  for (let i = 0; i < dataset.length; i++) {
    switch (dataset[i][0].substring(5, 7)) {
      case "01":
      case "02":
      case "03":
        dataset[i].push(dataset[i][0].substring(0, 4) + " Q1");
        break;
      case "04":
      case "05":
      case "06":
        dataset[i].push(dataset[i][0].substring(0, 4) + " Q2");
        break;
      case "07":
      case "08":
      case "09":
        dataset[i].push(dataset[i][0].substring(0, 4) + " Q3");
        break;
      case "10":
      case "11":
      case "12":
        dataset[i].push(dataset[i][0].substring(0, 4) + " Q4");
        break;
      default:
        break;
    }
  }
  return dataset;
}

hope this do the work.