Not totally sure what this excercise is meant to help reader learn. May not need it and might instead replace with excercise I describe above.

Updated exercise

I would just do the layering excercise when we introduce layering above. And not worry about the remainder of this excercise as I'm not sure much is learned from it. Also I would use the color 'firebrick' rather than 'red'. Basically the layering excercise should ask people to change the color of the points to firebrick, make them a bit bigger and also play with layering them above and below the line.