Graphr, the JavaScript Graphing Calculator
Check out the web-based demo at http://www.graphr.org/
This is a JavaScript implementation of a graphical calculator, with support for standard algebraic functions in the Cartesian plane. Features include fast, accurate graphing and full mouse zoom and pan support. Additionally, there are many graphing tools available, such as tracing a function, finding a root, finding the derivative, etc.
Designed for demonstration purposes only - https://www.desmos.com/calculator is a better calculator for real mathematics.