html, body, #container {

    width: 100%;

    height: 100%;

    margin: 0;

    padding: 0;

}

anychart.onDocumentReady(function () {

    var chart = anychart.area();

​

    var series = chart.splineArea([

        {x: 'January', value: 125},

        {x: 'February', value: -132},

        {x: 'March', value: 141},

        {x: 'April', value: -158},

        {x: 'May', value: 133},

        {x: 'June', value: -143}

    ]);

​

    // Set negative stroke.

    series.negativeStroke(function () {

        return anychart.color.setThickness(anychart.color.darken(this.sourceColor), 5);

    });

​

    chart.title('Set negative stroke parameter as a function');

    chart.container('container');

    chart.draw();

});