label (labelFunction, enableLabels) {
        if (!arguments.length) {
            return this._label;
        }
        this._label = labelFunction;
        if ((enableLabels === undefined) || enableLabels) {
            this._renderLabel = true;
        }
        return this;
    }
Set or get the label function. The chart class will use this function to render labels for each
child element in the chart, e.g. slices in a pie chart or bubbles in a bubble chart. Not every
chart supports the label function, for example line chart does not use this function
at all. By default, enables labels; pass false for the second parameter if this is not desired.
@example
// default label function just return the key
chart.label(function(d) { return d.key; });
// label function has access to the standard d3 data binding and can get quite complicated
chart.label(function(d) { return d.data.key + '(' + Math.floor(d.data.value / all.value() * 100) + '%)'; });
@param {Function} [labelFunction]
@param {Boolean} [enableLabels=true]
@returns {Function|BaseMixin}