grid not respecting row selection mode

Ive initialised my grid using MVC helper and set the selection mode to row but this is not being respected when the grid is rendered, if I hover over the grid , all the rows are 'selected'.  Im populating my grid via an ajax call (im populating a graph and grid from the same datasource).  Why is single row selection not being honoured ?

heres the grid definition

                                                    .Columns(column =>
                                                        column.For(x => x.PriceDateString).HeaderText("Price Date").Width("25%");
                                                        column.For(x => x.MaximumnPrice).HeaderText("High (£/MWh)").Width("25%");
                                                        column.For(x => x.AveragePrice).HeaderText("Average (£/MWh)").Width("25%");
                                                        column.For(x => x.MinimumnPrice).HeaderText("Low (£/MWh)").Width("25%");
                                                    .Features(features =>

and the ajax call to populate it

function Populate() {
var dayOption = $("input[name='optDays']:checked").val();
url: '@Url.Action("GetpriceData", "Price")',
data: { period: dayOption },
async: true,
type: 'GET',
success: function (returnval) {

if (returnval.success == true) {

dataSource: returnval.Records
$("#Grid").igGrid("option", "dataSource", returnval.Records);
if (returnval.success == false) {
bootbox.alert({ title: '<div class="text-center text-danger"><i class="fa fa-exclamation-triangle"></i>&nbsp;&nbsp;ERROR</div>', message: returnval['responseText'] });
error: function (returnval) {


both the chart and grid are populated exactly as i expected but the grid will not honour single row selection, whats going on here ?  anyone help ?