When I teach the MATLAB graphics class for new technical support engineers at MathWorks, I have one major exercise for them to do. This is that exercise. This is a good exercise as it involves a lot of skills:

• Lines only have one color
• Using NaN in plotting
• Using SET
• Using convenince functions like XLIM
• Difference between PLOT and LINE
• Setting callbacks like WindowButtonMotionFcn
• Managing state
• Using currentpoint property of an axis
• Using a create function and an update function
• Choosing good architecture to simplify implementation

There is a lot going on in this example, I believe that if someone can accomplish this exercise on their own, in a clean, well formated manner, they are well versed for many graphics challenges in MATLAB.