@duncan
No, showing data when sum({sessions}) <> 0 was not requirement.
Issue is
"Nesting of aggregate functions like SUM and SUM is not allowed. "
I think we can’t pass Aggregate function to another when using “periodToDateSum”. May be “periodToDateSum” internally aggregates.
for example
periodToDateSum(sum({total_orders}), day, MONTH, now())
Found another function in docs which allows to pass agg. functions, that solved the issue.
periodToDateSumOverTime(sum({total_orders})/sum(sessions), day, MONTH)
Thanks for your help @duncan