Confused about how to work around fl graphs and provider

Question

I'm trying to create an app where there is a button, and when a user clicks on that button, that day's timestamp is recorded and is uploaded in firestore. And then I fetch the data from there and update a fl chart widget.

This is how I get the data from firebase, convert it to DateTime and send it to the graph building method.

Future<void> updateOfflineGraph(String uid) async {
    //GraphData graphData = globalContext.watch<GraphData>();
    GraphData graphData = GraphData();
    List<dynamic> today = List<dynamic>();
    List<DateTime> now = List<DateTime>();
    try {
      DocumentSnapshot _docSnapshot =
          await _firestore.collection("graph").document(uid).get();
      today.addAll(_docSnapshot.data()['dates']);

      for (int i = 0; i < today.length; i++) {
        now.add(DateTime.fromMicrosecondsSinceEpoch(
            today[i].microsecondsSinceEpoch));
      }
      graphData.buildGraph(now);
    } catch (e) {
      print(e);
    }
  } 

And this is how I process the data for the fl chart.

class GraphData with ChangeNotifier {
  List<FlSpot> _data = List<FlSpot>();
  List<int> _date = List<int>();

  List<FlSpot> get data =>
      List.unmodifiable(_data..sort((a, b) => a.x.compareTo(b.x)));

  void addPoint(FlSpot spot) {
    _data.add(spot);
    notifyListeners();
  }

  void buildGraph(List<DateTime> dateTime) {
    removeAllPoints();

    int days = 0;
    int month = dateTime[0].month;
    for (int i = 0; i < dateTime.length; i++) {
      if (dateTime[i].month == month) {
        days++;
      } else {
        addMonth(days, month);
        days = 0;
        month = dateTime[i].month;
        i--;
      }
    }
    addMonth(days, month);
  }

  void addMonth(int days, int month) {
    addPoint(FlSpot(month.toDouble(), days.toDouble()));
    print(days);
    print(month);
    print(_data.length);
  }
}

And then do this to update the graph.

LineChartBarData(
                  spots:
                      // [
                      //   FlSpot(1, 3),
                      //   FlSpot(2, 4),
                      //   FlSpot(4, 5.5),
                      //   FlSpot(5, 1),
                      //   FlSpot(8, 4),
                      //   FlSpot(12, 3),
                      // ],
                      context.watch<GraphData>().data.length > 0
                          ? context.watch<GraphData>().data
                          : [FlSpot(0.0, 0.0)],
}

The problem that I'm facing is that when I do this whole thing, the points are stored in the _data list successfully, but when I try to call spots in LineChartBarData, it says that _data is null when it is called. I don't know why is this happening as the data was there in an iteration before.