All of these graphs have extra plots in them that we haven't talked about. Axis plots and Grid plots help you see locations on the graph so you can understand it better.
This graph has its axes and grids showing. There are two of each. I set them all to different colors so you could see what made what. The grid lines are in red and blue, and the axes are in brown and green. Notice that most of the grid lines show up lighter in color. Only the grid line for zero, and for major increments, is the full deep color.
An axis plot is a line with tick marks, labels on each tick, and a legend telling what variable the labels are taken from. It works very much like a line plot.
Grids work a bit differently. A grid plot actually draws a set of line plots. Usually they are parallel, but they don't have to be. As with other plots, they have the freedom to be very flexible.
Why do they need to be this flexible? That's the subject of the next page.