If you need a quick graph of a function you can get it done in no time with Google Search, just search for a function definition followed by the word "graph" and the first result will be an interactive plot of the resulting function.

E.g.:

- "x^2 graph" http://goo.gl/hSHtO
- "sin(x) graph" http://goo.gl/Wuim2
- "cos(x^2) graph" http://goo.gl/3SQ3h