<DataGridPremium
  rows={data.rows}
  columns={COLUMNS}
  aggregationFunctions={{
    ...GRID_AGGREGATION_FUNCTIONS,
    firstAlphabetical: firstAlphabeticalAggregation,
  }}
  initialState={{
    aggregation: {
      model: {
        director: 'firstAlphabetical',
      },
    },
  }}
/>