A convenient way to set labels. Currently set values will be mixined with `value` if used as a setter.
function(idx, value) {

        idx = idx || 0;
        
        var labels = this.get('labels') || [];
        
        // Is it a getter?
        if (arguments.length === 0 || arguments.length === 1) {
            
            return labels[idx];
        }

        var newValue = _.merge({}, labels[idx], value);

        var newLabels = labels.slice();
        newLabels[idx] = newValue;
        
        return this.set({ labels: newLabels });
    }