This week I was looking aroun Xero to find out exactly how much revenue per customer we have had in the last few years. There isn't a report for that - unless you go to each customer and list invoices updating the start/end period.
I wanted a simple report that would list the Top [X] customers by revenue in a period. Lucky we only have a few customers so I could manually check the ten or so records for the top ones, but anyone with a bigger flow would need more than this.