Oh no! You don't have a graphing calculator on hand? What do you do?
You can enter your graph equation into Google, and it will act like your trusty TI-83! Try entering x/3, (x/2)^2, ln(x), cos(pi*x/200) and check it out. You can follow the graph of any of the lines using the color-code in the upper right hand corner and plot the coordinates. Zoom in/out controls are in the upper left.
Congratulations; you've now helped Google take one step closer to Wolfram Alpha.