OVR_How do you condition a graph/chart to define a top 20?

I am trying to attach a graph/chart on an OVR report that reflects the top 20 Suppliers.  I can find no documentation explaining how to accomplish this.  I can see how to display the top 20 suppliers in the filter, my grid is a detail line grid, so how do I identify the group the values should be totaled by, in this case, per supplier, but could be per company, per business unit, etc.?

Thank you in advance for any assistance.