In the problem statement, it says "Draw a diagram of 𝐸(𝑛) vs. 𝑃(𝑛), with 𝑛 being the parameter along the curve. This is called a Z-plot." If you type "z-plot energy" into duckduckgo (or google), you will find a blog post by me: https://blogs.fau.de/hager/archives/7504
The Z-plot is convenient as it shows several important metrics (energy, performance, energy-delay product) and these are constant along straight lines. Nice if you ask "Where is the best performance?" or "Where is the lowest energy?", or "What is the tradeoff between energy savings and performance loss?".