Problem getting overall usage without stacking group by

I’m trying to display the overall read_bytes and write_bytes to a lustre filesystem. Each OST (storage target) is being pushed in to influxDB with a slightly different timestamp (see http://wiki.lustre.org/Lustre_Monitoring_and_Statistics_Guide#Stats for the format I’m parsing).

This works well when I include a group by OST and stack the graphs, ie
SELECT non_negative_derivative(mean("read_bytes"), 10s) /10 FROM "usage" WHERE ("fs" =~ /^$Filesystem$/) AND $timeFilter GROUP BY time($__interval), "ost" fill(null)

but I’d just like the final overall value on a summary panel - Summed metrics doesn’t show real data seems to be a similar problem and I’ve tried using sum(“read_bytes”) and setting $__interval to a fixed value with no avail.

is it possible to get the final value somehow without needing to show each member of the stack?

Many thanks
Andrew