This is a Lookback API query example of getting all past snapshots instances of a specific artifact where PlanEstimate is greater than zero. Similarly, you can change the criteria or the artifact to create even more complex queries:
Notice this query is asking for: FormattedID = "US1" and "PlanEstimate > 0". It will receive only the past snapshot instances that meet this criteria:
https://rally1.rallydev.com/analytics/v2.0/service/rally/workspace/
<<Workspace Object ID>>/artifact/snapshot/query.js?find={
$and: [{"FormattedID":"US1"},{"PlanEstimate": {$gt: 0}}]}&fields=["PlanEstimate","_SnapshotDate","_SnapshotNumber"]&compress=true
<Please see attached file for image>