http://www.bing.com/travel/?cid=homenav&FORM=Z9LH9

Go to the "Flexible?" section.

Pick a route (I'm picking SEA-LAS).

Now you get a graph.

Click on a dollar amount.

Now you get a page with a bunch of fares. Near the top of the page, on the right side, is an image of a graph with the heading "Fare history". (Well, it's not working right now...it says there's no price history available, but that's a temporary glitch because it worked great last night )

If that's what you're doing and you don't see either graph pop up, please describe the steps you are following.