c3/src/api.transform.js

c3_chart_fn.transform = function (type, targetIds) {
    var $$ = this.internal,
        options = ['pie', 'donut'].indexOf(type) >= 0 ? {withTransform: true} : null;
    $$.transformTo(targetIds, type, options);
};

c3_chart_internal_fn.transformTo = function (targetIds, type, optionsForRedraw) {
    var $$ = this,
        withTransitionForAxis = !$$.hasArcType(),
        options = optionsForRedraw || {withTransitionForAxis: withTransitionForAxis};
    options.withTransitionForTransform = false;
    $$.transiting = false;
    $$.setTargetType(targetIds, type);
    $$.updateAndRedraw(options);
};