Under single-objective optimization, the objective value is continually improved over time (in this example, it is minimized). This is the top plot in the figure. Multi-objective optimization, on the other hand, attempts to optimize two or more objectives at once (in this example, two objectives are minimized). The bottom left plot shows both objectives over time. Rather than a single value, both objectives develop a range of values that trade off against each other. The final trade-off is shown in the bottom right plot.