Grafana only shows 35 days of data. I have replicated this in the native Graphite GUI. My first thought was that this was a problem in the storage-schemas.conf file, but I have visually verified that it was correct, and I have run whisper-info.py against the files to verify that they are correct.
I found the problem & am replying to my own post for archival search purposes. Because the sheer number of items polled from our Production cluster was causing polling misses, I had changed the "data_update_freq" in /opt/netapp-harvest/netapp-harvest.conf to 180 seconds. I assume because this was not an even multiple of 5 minutes, the longer term retention in the whisper database files for this cluster was not getting generated properly. I let the cluster default back to 1 minute polling intervals, and am eating the missed polls. The grafana graphs now work correctly, though.