This question uses a pivot table to display your total spend per supplier per month.

Once you have built the question and visualized it, click on the Settings button at the bottom of the screen and then on Pivot Table Options. Drag the Date Invoice field into the table columns area like so:

A nice feature of a report like this is that you can click on any cell in the table to drill down in to the data. It will generate a new report with the individual transactions for that supplier in that period.