http://cl.ly/image/0G2k2M360F0B

custom_chart.js

var pieChart = A.Widget.getByNode($0);
var graphic = pieChart.get("graph").get("graphic");
var width = graphic.get('width'), height = graphic.get('width');
var newWidth = width - 100, newHeight = height - 100;
var circle = graphic.addShape({type: 'circle',
    width: newWidth,
    height: newHeight,
    x: 100,
    y: 100,
    fill: {color: '#ffffff'},
    stroke: {color: '#555555', width: 1}
});

var xy = graphic.getXY();
diff = 40;
xy[0] += diff/2;
xy[1] += diff/2;
circle.setXY(xy);
circle.set('width', width - diff);
circle.set('height', height - diff);
circle.set('fill', {type: "radial",
        stops: [
           {"color":"#555","opacity":1,"offset":0},
           {"color":"#000","opacity":1,"offset":1}
        ],
        cx: 0.5,
        cy: 0.5,
        fx: 0.48,
        fy: 0.02,
        r: 0.49,
        rotation: 0
});