Araneus diadematus
(Cross Orb-weaver)

Hi, she was an Orbweaver in family Aranidae. Her ventral (underside) markings appear to be those of genus Larinioides. There are three species in that genus in Wisconsin: L. cornutus, L. patagiatus, and L. sclopetarius. The dorsal markings are needed to tell them apart. Yes, Charlotte was an orbweaver (Araneus cavaticus), her species was given in the story though she was fictional. 🙂