How to make subcategories in yii2 select menu widget?

Question

Yii

using select2 widget:

echo $form->field($model, 'date')
->widget(kartik\select2\Select2::className(), 
['data' => \yii\helpers\ArrayHelper::map(\frontend\models\Data::find()->All(), 'date', 'date')]);

how to make popup subcategories: