Use the the "Compute Cost" in Y-Axis in EC2 instance report:
Savings Plans (SPs) and Reservation Instances (RIs) can only apply to compute costs. The EC2 Instance Cost History report by default is looking instance costs which contains compute + transfer costs and transfer costs will show as On-Demand. RIs don't apply to transfer costs. The cleanest way to see Savings Plan - Unused costs is to use "Compute Cost".