Which chart is commonly used to illustrate categories and their values in relative proportions?

A pie chart is the ideal choice for illustrating categories and their values in relative proportions. It visually represents different segments as slices of a whole, making it easy to compare the sizes of each category relative to the sum. Each slice corresponds to a category, showing its contribution to the total in a straightforward manner. This is particularly effective for displaying a small number of categories where the differences in proportions can be easily distinguished.

In this context, while a bar chart can also show data values for different categories, it presents this information in a linear format, making it less effective than a pie chart for depicting relative proportions visually. Similarly, line charts are primarily used for showing trends over time rather than comparing relative sizes of categories. Scatter plots are focused on showing relationships between two numerical variables, making them unrelated to the concept of illustrating category proportions. Thus, the pie chart stands out as the most effective tool for conveying relative proportions among various categories.