GraphCalc3DS by flarn2006 is a simple graphing calculator that works with RPN equations.

Release notes:

This new release includes a new feature that several people have requested: RPN input is now optional. Pressing A will open the system keyboard, where you can type in an equation using traditional algebraic notation. In addition, if you have a New 3DS (or possibly a Circle Pad Pro, not sure if it’s compatible) you can control the first two variable sliders (‘a’ and ‘b’) by moving the C-stick.