The plot on the left is the graph of f.
Dragging the red dot along the x-axis displays the cumulative area
under the graph. At the same time, the plot on the right displays
the graph of the cumulative area, that is, the graph of F, where
x is the current position of the red dot. In other words, the height
of the blue
line on the right is equal to the area of the blue region on the left.