The OpenNMS Flows API inserts "NaN" for any cell in the result set that doesn't have a value.
Grafana's reduce expressions will always return "NaN" if any values are null or NaN.
Helm should provide a way to filter "NaN" from the results returned by OpenNMS, so reducing expressions can be applied to them.
Any data set that includes "NaN" cannot be reduced.
Could add a nonNull() query transform, similar to the groupByInterval() one.