"Net present value versus return on investment" is not a common pair of variables in visual bubble chart displays of the balance of a portfolio. Bubble charts typically display variables such as risk versus reward, cost versus benefit, and ease versus attractiveness. These pairs help in visualizing trade-offs and strategic decisions in a portfolio. NPV and ROI, while important financial metrics, are less commonly paired in bubble charts because they are both measures of financial return rather than orthogonal attributes like risk, cost, or ease.