var updateChart = function updateChart () {
  $.getJSON( "URL.json", function( data ) {
    // callback for event to take place after JSON has been retrieved.
    // data is the object containing the JSON.
  });
};

var init = function init () {
  updateChart();
};

var cronJob = {
  interval: 0,
  createInterval: function() {
    this.interval = setInterval(updateChart, 500);
  },
  clearInterval: function() {
    clearInterval(this.interval);
  }
};

init();