Which type of chart is least effective for displaying changes over time?

A pie chart is primarily designed to show the proportions of a whole at a single point in time rather than illustrating changes over a period. Their circular format effectively communicates how different parts compare to the whole, making them suitable for showing percentage distributions; however, they do not provide a clear view of trends, variations, or changes over time.

In contrast, bar charts, line charts, and area charts are specifically structured to highlight temporal changes and trends. For example, a line chart is particularly effective at visualizing data points over intervals, making it easy to see rises, falls, and patterns over time. Bar and area charts can also effectively communicate time-series data by representing quantities over time, but they may not convey trends as directly as a line chart does.

Thus, when considering the goal of displaying changes over time, a pie chart is not suitable, making it the least effective option among those listed.