A plot graph is sometimes called a plot diagram or plot triangle.
The graph starts at the left and moves to the right, up to a point as if drawing the first side of a triangle, descending down to create the second side of the triangle, and then moving off to the right.
The first part of the graph is the introduction (or exposition). The next part is the rising action: these are the things the characters do that move the plot along and lead to the climax. The climax is listed at the top of the triangle, and this is often also the turning point: it is the part of the story with the greatest intensity. The second side of the triangle moves down and is called the falling action. This are the things that occur after the climax. The last part is called the resolution (denouement), where any of the loose ends are generally resolved.
The introduction in Frankenstein would list quotes from Walton's letters introducing his obsession to discover undiscovered territory in the northern climes (which will parallel Victor Frankenstein's obsessive desire to create life).
Rising action would indicate quotes that introduce the relationship of the characters (especially Elizabeth), Victor going away to the university, his experiments, the creation of the creature, the creature's struggles to survive, his pursuit of Victor, the murders, etc.
Sometimes different people will plot the climax at a different point in a story, but I feel it took place with Elizabeth's death.
It is at this point that Victor's life takes on the purpose solely of finding and killing the monster, and I would identify this as the first step of the falling action. The movement would continue with Victor's pursuit, clues left by the creature, Victor meeting Walton, Victor telling his story, Victor's death, and the creature's lament at the loss of his "father."
My sense of the resolution is Walton's change of heart in terms of endangering the lives of his men simply for his own personal gratification (in continuing their journey under extremely dangerous circumstances). He learns from Victor's mistakes and turns his ship homeward.