Generate a Pie chart with expression rule

In case you are not clear with the above answer please refer to the code below.

a!localVariables(
  local!selectedDepartment,
  /* This query fetches the grouped data for the pie chart. */
  local!chartDatasubset: a!queryEntity(
    entity: cons!EMPLOYEE_ENTITY,/*entity name */
    query: a!query(
      aggregation: a!queryAggregation(
        aggregationColumns: {
          a!queryAggregationColumn(
            field: "department",
            isGrouping: true
          ),
          a!queryAggregationColumn(
            field: "id",
            alias: "id_count",
            aggregationFunction: "COUNT"
          )
        }
      ),
      pagingInfo: a!pagingInfo(
        startIndex: 1,
        batchSize: -1,
        sort: a!sortInfo(
          field: "department",
          ascending: true
        )
      )
    )
  ),
  {
    a!pieChartField(
      series: a!forEach(
        items: local!chartDatasubset.data,
        expression: a!chartSeries(
          label: fv!item.department,
          data: fv!item.id_count,
          /* Clicking on the chart sets the filter value for the grid. */
          links: a!dynamicLink(value: fv!item.department, saveInto: local!selectedDepartment)
        )
      ),
      showWhen: isnull(local!selectedDepartment), 
      colorScheme: "MOSS",
      style: "DONUT"
    )
  }
)