creating highchart with ajax json data

I think you cannot return values from the success call instead you would need to call a function instead. So set up your function that initializes your chart, and in the ajax success call that function with the data

With your code example

function visitorData (data) {
   $('#chart1').highcharts({
    chart: {
        type: 'column'
    },
    title: {
        text: 'Average Visitors'
    },
    xAxis: {
        categories: ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday']
    },
    yAxis: {
        title: {
            text: 'Number of visitors'
        }
    },
    series: data,
  });
}
$(document).ready(function() {
 $.ajax({
    url: '/visitdata',
    type: 'GET',
    async: true,
    dataType: "json",
    success: function (data) {
        visitorData(data);
    }
  });
 });