Google cloud bigquery edit sql fill missing point as zero

i can get 4 point data from bigquery, but i want to show a continuously line, $__timeGroup doesn’t work to fill missing point as zero.
my query is

  $__timeGroup(started_at, 1d, 0),
  CAST (service_name  as String ) AS metric,
  `count` as count
  and service_name="xxx"
order by started_at asc  

Maybe ask the plugin author on the GitHub project page?