Unable to graph data from node_exporter stored in newts indexed by label

We are evaluating Horizon 27.1.1.

We are using SNMP and node-exporter to collect data.

We are using newts to store the timeseries data.

On the Resource Graphs page for a server node I can see sections for 'Disk' and 'Filesystem'.

The sections list all the disks and mountpoints but when I try to generate a graph for any of them, all I get is a page reporting "There is no data for this resource".

When I look in newts I can see the data is there.

For example, here is some data from the samples table for the sda device:

      G | 1620864000 |          snmp:fs:ID26:1618997123848:nodeExporterDisk:sda:node_exporter_disks | 2021-05-13 15:03:09.327000+0000 |          write_time_seconds_total |       null | 0x01000000000002ec44
      G | 1620864000 |          snmp:fs:ID26:1618997123848:nodeExporterDisk:sda:node_exporter_disks | 2021-05-13 15:03:09.327000+0000 |            writes_completed_total |       null | 0x01000000000aceefa1
      G | 1620864000 |          snmp:fs:ID26:1618997123848:nodeExporterDisk:sda:node_exporter_disks | 2021-05-13 15:03:09.327000+0000 |               writes_merged_total |       null | 0x010000000000f14b6a
      G | 1620864000 |          snmp:fs:ID26:1618997123848:nodeExporterDisk:sda:node_exporter_disks | 2021-05-13 15:03:09.327000+0000 |               written_bytes_total |       null | 0x01000014fec2722c00

 in it you can see the resourceType name "nodeExporterDisk", the result of thr group-by-exp "sda" , and the collection group name "node_exporter_disks".

Is this a bug or am I misunderstanding something?