events: {
    load: function() {
        // set up the updating of the chart each second
        var series = this.series[0];
        setInterval(function() {
            $.getJSON('db.php', function(data) {
        }, 1000);

1st, series is an array, so you'll want series[0]. 2nd, you need to use setData to change the series data after the chart is drawn (

It's not clear from your code if you're already doing it, but you will need to set the original series : [{data:[...]}]

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