In my experience, the data usage resets every time you switch your SIM. If you get a prepaid SIM in the U.S. then it should only show the data you used here. I just got back from England and France and the local SIMs I used in each country caused the data to reset automatically.
When I got back home, it now shows my data usage for the whole month on my local Sim, but the graph is flat for the two weeks I was gone.
If you are using roaming data instead, you can just drag the sliders for the period you want to track.