Can anyone give me the site from where I can download the software for plotting all kinds of graphs?
Another one is WinPlot: http://math.exeter.edu/rparris/winplot.html. Kind of an annoying background on the web page, but the software is good & it's free.
The above suggestions are great. On the off chance you have a mac though you will automatically have a program pre-installed which you wouldn't necessarily know about. Spotlight for Grapher, it's simple and easy to use but has some really nice advanced stuff it can do too.
There's a nice one called 'grapes'.
GRAPES : Function Graphing Software
There's Graphmatica for Windows. It's shareware, but I understand it is free for those who really can't afford even that. Perhaps another way of asking for a donation for the effort. It's a great 2D program by the way.
The Apple iMac has "Grapher" bundled with a ton of other useful software as part of the package.