<?php
    $array_sellers = array(
        array(
            'agent'=>1,
            'date' =>'2019-01-01',
            'sales' =>2
            ),
        array(
            'agent'=>1,
            'date' =>'2019-01-02',
            'sales' =>4
            ),
        array(
            'agent'=>2,
            'date' =>'2019-01-03',
            'sales' =>1
            ),
        );
    $array_dates = array('2019-01-01','2019-01-02','2019-01-03');
    $output =  array(
        array(
            'agent'=>1,
            'date' =>'2019-01-01',
            'sales' =>2
            ),
        array(
            'agent'=>1,
            'date' =>'2019-01-02',
            'sales' =>4
            ),
        array(
            'agent'=>1,
            'date' =>'2019-01-03',
            'sales' =>0
            ),
        array(
            'agent'=>2,
            'date' =>'2019-01-01',
            'sales' =>0
            ),
        array(
            'agent'=>2,
            'date' =>'2019-01-02',
            'sales' =>0
            ),
        array(
            'agent'=>2,
            'date' =>'2019-01-03',
            'sales' =>1
            ),
        );

?>