Edit in JSFiddle

var bigData = [
    {
        data: [[0, 6]],
        label: 'a',
        sub: [{   // sub pie data
            data: [[0, 2]],
            label: 'a1'
        }, {
            data: [[0, 4]],
            label: 'a2'
        }]
    }, {
        data: [[0, 4]],
        label: 'b',
        sub: [{   // sub pie data
            data: [[0, 3]],
            label: 'b1'
        }, {
            data: [[0, 1]],
            label: 'b2'
        }]
    }
];

(function basic(container, data) {
    var graph;

    // Draw Graph
    graph = Flotr.draw(container, data, {
        title: 'Click a slice to see the details',
        pie: {
            show: true
        },
        mouse: {
            track: true,
            trackFormatter: function (obj) {
                return obj.series.label + ': ' + obj.y;   
            }
        }
    });
    
    graph.observe(container, 'flotr:click', function (position) {
        var obj = position.hit; // hit information when there is a hit.
        if (obj && obj.series && obj.series.sub) {
            return basic(container, obj.series.sub);
        } else {
            return basic(container, bigData);   
        }
            
    });
    
})(document.getElementById("example"), bigData);

<div id="example"></div>

#example {
    width: 400px;
    height: 300px;
}