iGraphmatica

Graphmatica is a powerful, easy-to-use, equation plotter with numerical and calculus features:

• Graph Cartesian functions, relations, and inequalities, plus polar, parametric, and ordinary differential equations.
• Data plotting and curve-fitting.
• Up to 999 graphs on screen at once.
• Numerically solve and graphically display tangent lines and integrals.
• Find critical points, solutions to equations, and intersections between Cartesian functions.
• AutoSquare keeps your graphs from getting squashed as you zoom or enable helper panes; or disable it in General Settings so you can scale each axis independently by pinching horizontally or vertically.
• Print your graphs, copy to clipboard or export to Photo Album.
• Share graph documents with friends or Graphmatica for Mac or Windows via email or iTunes.
• Online help and demo files make getting up to speed a snap.

In summary, a great tool for students and teachers of anything from high-school algebra through college calculus.