A plot graph in literature is really a chart or a diagram that is intended to visually represent the key events that occur a story or book.
It is created by drawing what looks like a modified bell curve and labeling the different plot parts with the plot terms of exposition, rising action, climax, falling action, and denoument. Check out the ReadWriteThink link below to see what this looks like.
Usually, a teacher will give out a diagram like this with spaces in each area for students to fill out with important events from each period.
The exposition is the beginning of a story, sometimes called the basic situation.
The rising action is often the longest part of a story, with events and conflicts that change and intensify.
The climax is the big moment when the primary conflict(s) is resolved.
The falling action leads the reader/viewer to the end as the events precipitated by the climax play out.
Finally, the denoument, also called the resolution, finishes up the story.
There is also another kind of plot diagram that is used in statistics to plot points generated by a specific statistical function. This kind of diagram has nothing to do with literature, of course. Its purpose is to show how many points occur in certain areas of concern with a statistical problem or idea.