Jacques Bertin, a French cartographer and theorist, introduced a set of visual variables that are fundamental to the design and interpretation of visualizations. These variables are essential for encoding data in a way that can be easily understood by viewers. Bertin identified seven primary visual variables:

• Position: This refers to the placement of elements within a visual space (such as a graph or map). Position is one of the most powerful visual variables because the human eye is very sensitive to spatial relationships. It is often used to represent quantitative data.

• Size: Size indicates differences in quantity or magnitude. Larger or smaller symbols can convey information about the relative importance or value of data points.

• Shape: Different shapes can be used to categorize or differentiate types of data. Shape is particularly useful for representing nominal data, where items are grouped into distinct categories.

• Value: This refers to the lightness or darkness of a color. Variations in value can be used to represent ordered data, such as ranking or magnitude.

• Color (Hue): Color is effective for distinguishing categories or groups. Different hues can represent different qualitative categories, but they should be used carefully to ensure they are distinguishable to all viewers, including those with color vision deficiencies.

• Orientation: This pertains to the direction in which a symbol is pointed or aligned. Orientation can be used to represent directional data or to differentiate between categories.

• Texture: Texture involves the pattern or density of lines and marks. It can be used to distinguish between areas or categories, often in conjunction with other visual variables.

Bertin's visual variables are foundational in the field of data visualization because they provide a systematic approach to encoding data graphically. By effectively using these variables, designers can create clear, informative, and aesthetically pleasing visualizations that enhance the viewer's understanding of the data.