Have you noticed that we have added Discount as column in the Data extraction: Invoices report?
The Data extraction: Invoices report provides a total overview of your invoices and what you have invoiced incl. the VAT amount. It is also possible to see if you have given a discount on an invoice. You find the field to the right in the report.
The results list in Data extraction: Invoices looks like this, and you find the discount field to the right.
The report is good to use, if you want to export your invoice data for further analysis in e.g. Excel.