I'm not explaining this clearly. Consider this SQL query from the sample dataset:
select day_of_month(created_at), sum(subtotal),avg(subtotal)
group by day_of_month(created_at)
This makes a chart that looks like the first below.
However, I want the green line to be on a separate Y axis - the second image shows what can be done with the same raw data in excel (average is on the right-hand axis):
Does this make sense?