how can i implement real-time fl_chart flutter?

Question

This is fl_chart i want to implement realtime graph for eg., heart rate against time.

child: StreamBuilder<QuerySnapshot>(
                  stream: FirebaseFirestore.instance
                      .collection('#')
                      .orderBy("ad")
                      .limitToLast(5)
                      .snapshots(),
                  builder: (context, snapshot) {
                    if (snapshot.hasData) {
                      return LineChart(

                        mainData(snapshot.data),

                      );
                    }

this the real time data where the data being constructed

LineChartData mainData(QuerySnapshot graphData) {
    var liveGraphData;
    liveGraphData = graphData.docs
        .map((documentSnapshot) => Gadget.fromMap(documentSnapshot.data()))
        .toList();

    List<FlSpot> gatherGraphData() {
      List<FlSpot> graphPoints = [];
      for (int i = 0; i < liveGraphData.length; i++) {
        graphPoints.add(
            FlSpot((liveGraphData[i].ts).toDouble(), liveGraphData[i].yadc));
      }
      print(graphPoints.length);
      return graphPoints;
    }