Influxdb and grafana: howto create series that is the sum of two other series
In theory, switch Grafana query editor to the raw mode and write a query where you use a subquery - outer query will just sum data precalculated by inner query:

SELECT SUM(field_from_main_query) FROM (
  &lt;your current Grafana query with GROUP BY time(10s) hostname&gt;
) GROUP BY time(10s)