How to display selected month data from flutter dropdown selection using json

I have try to display Selected month data in dropdown selection in flutter , supposed I was select January then it display the January month record in json formate

String monthWiseGraph;
Future<String> getMonthlyGraphData() async {
String url = 'http://example.com/getAgentMonthGraph.php?month='+ monthWiseGraph ;
var response = await http.get(url);
return response.body;
}
Container(
        height: 130,
        width: 200,
        padding: EdgeInsets.only(left: 30, top: 50),
        child: Card(
          // color: Colors.blueGrey,
          shape: RoundedRectangleBorder(
              borderRadius: BorderRadius.only(
                  topRight: Radius.circular(20),
                  bottomLeft: Radius.circular(20)),
              side: BorderSide(color: Colors.black)),
          child: Column(
            children: <Widget>[
              Container(
                padding: EdgeInsets.only(top: 10, right: 20, left: 20),
                child: DropdownButtonHideUnderline(
                  child: DropdownButton(
                    hint: Text("Monthly",
                        style:
                            TextStyle(color: Colors.black, fontSize: 25)),
                    value: monthWiseGraph,
                    icon: Icon(Icons.arrow_drop_down),
                    iconSize: 30,
                    elevation: 16,
                    style: TextStyle(color: Colors.black),
                    onChanged: (String monthNewValue) {
                      setState(
                        () {
                          monthWiseGraph = monthNewValue;
                          print(monthWiseGraph);
                          _showMonthlyAlert();
                        },
                      );
                    },
                    items: <String>[
                      'January',
                      'February',
                          .
                          .
                      'December'
                    ].map<DropdownMenuItem<String>>((String value) {
                      return DropdownMenuItem<String>(
                        value: value,
                        child: Text(
                          value,
                          style: TextStyle(
                            fontSize: 20,
                          ),
                        ),
                      );
                    }).toList(),
                  ),
                ),                  
              ),
            ],
          ),
        ),
      ),

This is My January data

[
  {
      userId: "2",
      name: "Manish",
      total: "32"
     },
    {
       userId: "3",
      name: "Altaf",
      total: "31"
       }
  ]

This is My February data

[
  {
    userId: "4",
    name: "Prathamesh",
    total: "68"
  },
  {
   userId: "7",
   name: "Aniket",
   total: "62"
  },
  ]

Above image I display the selected month data When I select january from dropdown then it display january data from json like manish and altaf and when I select february it dipslay Prathamesh and Aniket data below is this is normal graph I am new to flutter

This is my drop down