The best math graphing tool you will ever use online or offline, period.
We have used desmos graphing tool with our SuprAcademy students when covering topics ranging from linear equations to exponential growth. It is easy to use, light, fast and beautiful. It supports graphing multiple graphs in the same frame, allowing for visual demonstration of different concepts such as function transformations and graphically explaining the difference among various functions.
Also available for download for offline use as an app for your Android/iPhone.