There is a yellow graph on the screen when I am reviewing the pictures. How do I remove it?

Make sure Image Review is set in Custom Settings. See page 90 of the manual: http://www.fixya.com/support/p418-nikon_d50_digital_camera_with_18_55mm/usermanual-D690EED-pdf

While viewing your picture, press the INFO button repeatedly to cycle through the different available views.

If you were seeing a preview on the LCD monitor on the back of the camera, then there was something seriously wrong with the camera. Nikon did not introduce the LiveView feature until later models.
If you're asking about review instead of preview and seeing graphs or lots of data, press up/down on the multiselector to cycle through different views of your pictures until you get to one you like.

On most camera, pressing up or down on the circular multiselector will let you cycle through the different views of your picture. If this doesn't do it, please reply to this post stating the make and model of your camera so someone can give you specific instructions.

You're seeing a histogram. You can press up/down on the multiselector to cycle through the different views.

You're seeing the histogram, which shows valuable information about your picture's exposure. While viewing, press up/down to cycle through the different views of the picture.

This is a convergence failure --you will need a technician to repair the circuit

Bring up the photo on the camera. Press up or down until you get the histogram (yellow graph). When you get this, press up once, and look at the bottom of the screen.

Look for a button on the back of the camera saying "info" or something similar. Pressing the button will step you through a few different information screens along with not showing information from the screen.

Press up or down on the 4-way command button to toggle between histogram, shooting info, custom settings, highlights, or none.