Sorry, I missed this question somehow earlier. Harvest creates a summary of volume metrics for each aggregate, for each node. However the actual volume metrics are only emitted under SVM's. This is only done because we don't want to store the same data twice and in the Whisper/Graphite database there is not other way around. I could modify the plugin that does this (/opt/netapp-harvest/plugin/cdot-volume) to store them under the aggregates as well if you don't mind having redundancy in your database, but I probably won't find time until end of next week.
This kind of stuff will be much better solved in Harvest 2.0 that will use Prometheus as its default database (Graphite/Whispler will be supported as well). In Prometheus we will not need to store the same data twice, we will just use labels to associate volumes with SVMs, nodes, aggregates, etc.