Calca is one of my favorite calculators for iOS, Mac, and Windows. Calca for iOS just updated with function graphing. Just like the desktop versions (see this link for more info), the graphs on iOS are pretty and functional. Calca is like the Markdown of math. I just type notes, numbers and equations. Calca does the hard stuff.
The iOS version is currently free but has a new patronage option through in-app purchase. I think it's totally worth another purchase. I keep calculation templates and financial summaries in Calca. If I ever need to convert U.S. dollars to Oku Yen, Calca is where I go. Just try that as a Google or Wolfram Alpha query.