NZGrapher - Online
NZGrapher is a free graphing tool that can run on any device, without an install. NZGrapher was developed to run on anything with a browser, computers, iPads, ChromeBooks, Microsoft Surface, Android, even iPhones.

You can access it at

NZGrapher - At your school
Full details for getting NZGrapher running at your school are avaialbe here. NZGrapher has proved to be immensely popular, to the point that at times the server struggles with the load. If you would like to arrange to have NZGrapher hosted at your school (just for your own school's use, or shared with others if you want) this can normally be easily arranged, as most schools already have a server capable of running NZGrapher (you need a web server running PHP). If this is of interest to you please see full details here.

NZGrapher for Windows
If the server load is getting you down, and you would like to run NZGrapher on your own machine, you can do that if you are running windows.
There are some known issues: If all of that doesn't deter you, you can download NZGrapher.
Once installed it will notify you automatically of updates which you will be able to get with the press of a button, so you'll always have the most up to date version.

If you come across any issues with it please contact me.