You are wondering how we built this graph? Here's our method:
We sum all invoices issued in January and all payments received in January. We do not include any credit note issued during that same period. If one payment was created later in Upflow for some reason (in February for example) but was received in January, it will count in the January stats.
Last but not least, if you handle multi-currency, this graph will include any invoice or payment from another currency.
Let us know if you have any other questions!