Hi Jon,
here are some steps how to do that:
1. Create new report
2. Select Custom Chart
3. Change selection method to Specific Objects (static selection)
4. Change Show dropdown to Interfaces
5. Interfaces should be grouped by nodes so choose appropriate nodes and their interfaces will be automatically added to the selected list
6. Add to layout, add data series with property you want to graph
Hope this helps,
Zbynek Botlo