Do you have a manual? If you have worked any of the examples, especially the fast start examples right in the front of the book, they show how to make many interesting graphs.
I couldn't find my 84, but on my 83, this is what it took.
1) Turn it on.
2) Press the [Y=] button
3) Enter 4 next to the "\Y1="
4) Press the [Draw] button.
5) With any luck, your window and view settings will shot it to you.
Please read your owner's manual. There is a ton of useful stuff in there. Work ALL the examples. EVERY ONE.
Okay, just kidding. For a vertical line, get to the blank home screen and press [2nd][PRGM][ENTER]