display chart with chartJs, chartJs only when 2 id selected not showing result of one sided, otherwise single or multiple record selected works

Question

i am using this chart example where i updated query in data.php with

working

"SELECT student_id,student_name,marks FROM tbl_marks WHERE student_id IN(1, 2, 3)"

not working

"SELECT student_id,student_name,marks FROM tbl_marks WHERE student_id IN(1, 2)"

basically single entry and multiple entry works fine but when i select two data row it shows blank one side of bar graph.

<script>
        $(document).ready(function () {
            showGraph();
        });


        function showGraph()
        {
            {
                $.post("data.php",
                function (data)
                {
                    console.log(data);
                     var name = [];
                    var marks = [];

                    for (var i in data) {
                        name.push(data[i].student_name);
                        marks.push(data[i].marks);
                    }

                    var chartdata = {
                        labels: name,
                        datasets: [
                            {
                                label: 'Student Marks',
                                backgroundColor: '#49e2ff',
                                borderColor: '#46d5f1',
                                hoverBackgroundColor: '#CCCCCC',
                                hoverBorderColor: '#666666',
                                data: marks
                            }
                        ]
                    };

                    var graphTarget = $("#graphCanvas");

                    var barGraph = new Chart(graphTarget, {
                        type: 'bar',
                        data: chartdata
                    });
                });
            }
        }
        </script>