04_graphical_elements.Rmd

Graphical Elements of a Plot
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Marks and Channels
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A data visualization is useful only if it encodes information in a way that our
eyes can perceive and our brain can understand. **Marks** and **channels** are
the building blocks of all data visualizations and are employed to accomplish
this encoding. 

**Marks** are the the basic geometries, or graphical elements, in a plot that
depict our data items or their linkages. Marks indicate "where" something is
and include points (0d), lines (1d), areas (2d), and volumes (3d).

**Channels** are the attributes of that control how the marks appear. Channels
are used to encode (or indicate) the values or meaning of our data. Channels
were first described in the mid-20th century by Jacques Bertin in his book
*Semilogie graphique* (the *Semiology of Graphics* [1967]), which argues that
visual perception operates according to rules that can be followed to express
information visually in intuitive, accurate and efficient ways. He described
seven main categories of visual variables (channels): location or position,
size, shape, orientation, color, and texture. More recent publications list up
to 12 channels useful for encoding meaning in data visualizations ([Roth
2017](https://doi.org/10.1002/9781118786352.wbieg0761)). 

By understanding the nature of our data in combination with the principles of
visual perception, we can decide which marks and channels to use for a given
data visualizations.

![](./img/workshop_data_visualization_munzer_marks_channels_figure.PNG)
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*[Image from 'Visualization Analysis and Design' by Tamara Munzner](https://www.oreilly.com/library/view/visualization-analysis-and/9781466508910/K14708_C005.xhtml)*
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