Graphite

Graphite is an efficient and accurate graphing calculator for the iPhone and iPod touch. Graphite combines an intuitive, polished multi-touch interface with powerful numeric methods to allow fast and precise computation of roots, extrema, integrals, and intersections. Scale axes independently using the custom pinch gesture, then trace along curves for values using as many fingers as you want!

Graphite lets you easily manage as many equations as you need and quickly enter new ones using the dedicated equation input screen.

If Graphites helpful yet unobtrusive usage instructions and error dialog boxes still leave some aspect confusing, it also holds an extensive onboard help document.