[Solved] Google LineChart Error: Data column(s) for axis #0 cannot be of type string

i have run my google line chart and following error occure on my webpage:

error: Data column(s) for axis #0 cannot be of type string.

please someone can me help me.

my code is here:

Javascript: 

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript" src="https://www.google.com/jsapi"></script>
<script type="text/javascript">
    google.load("visualization", "1", { packages: ["corechart"] });
    google.setOnLoadCallback(drawChart);
    function drawChart() {
        var options = {
            title: 'ipaddress',
            width: 600,
            height: 400,
            bar: { groupWidth: "95%" },
            legend: { position: "none" },
            isStacked: true
        };
        $.ajax({
            type: "POST",
            url: "WebForm1.aspx/GetChartData",
            data: '{}',
            contentType: "application/json; charset=utf-8",
            dataType: "json",
            success: function (r) {
                var data = google.visualization.arrayToDataTable(r.d);
                var chart = new google.visualization.LineChart($("#chart")[0]);
                chart.draw(data, options);
            },
            failure: function (r) {
                alert(r.d);
            },
            error: function (r) {
                alert(r.d);
            }
        });
    }
</script>
<div id="chart" style="width: 900px; height: 500px;">
</div>

C#

namespace WebApplication13
{
    public partial class WebForm1 : System.Web.UI.Page
    {
        [WebMethod]
        public static List<object> GetChartData()
        {
            string query = "SELECT [time],[average] FROM [dbo].[graph] WHERE ipaddress='192.168.15.167'";
            string constr = ConfigurationManager.ConnectionStrings["dbcs"].ConnectionString;
            List<object> chartData = new List<object>();
            chartData.Add(new object[]
            {
                "time", "average"
            });
            using (SqlConnection con = new SqlConnection(constr))
            {
                using (SqlCommand cmd = new SqlCommand(query))
                {
                    cmd.CommandType = CommandType.Text;
                    cmd.Connection = con;
                    con.Open();
                    using (SqlDataReader sdr = cmd.ExecuteReader())
                    {
                        while (sdr.Read())
                        {
                            chartData.Add(new object[]
                            {
                                sdr["time"], sdr["average"]
                            });
                        }
                    }
                    con.Close();
                    return chartData;
                }
            }
        }
    }
}

datatable is like this:

ipaddress            time     average      pingtime

192.168.10.5       10:15     10           25/5/2021 10:15