Generate a Pie chart with expression rule

You can use the output directly by indexing your data from the query rule and applying loop on the chart series and again indexing the labels and counts.

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"
    )
  }
)

Thanks Ujjwal, I was able to generate the report 

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