Pie, Donut Charts and Stacked bar Graph

Visualising Proportions as a Whole

When your goal is to show the composition of a whole, where all parts sum up to 100%, Pie Charts and Donut Charts are the traditional, though often debated, visualization choices. Both chart types encode proportional data using angles, arcs, and area.

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1. Pie Chart: The Classic Whole

A Pie Chart divides a circle into sectors (slices), where the size of each slice is proportional to the quantity it represents. Like a pizza!

• Primary Purpose: To illustrate how a fixed whole (100%) is divided into several discrete categories.

• Visual Encoding: The proportion is encoded by the angle of the slice, the area of the slice, and the arc length along the perimeter.

• Best For:

  • Few Categories: They work best when there are 2 to 5 categories. Beyond this, slices become too thin and difficult to compare.

  • Highlighting Dominance: They are effective for instantly showing if one slice represents a majority (close to 50% or more).

  • Simple Part-to-Whole Relationship: When the primary message is simply "this chunk makes up this portion of the total."

• The Caveat (The Perception Problem): As noted by Cleveland & McGill's research, the human eye is poor at accurately judging angles and area (the primary encodings of a pie chart). This makes it very difficult to compare two slices of similar size (e.g., is 25% larger than 23%?). This is why bar charts are generally preferred for precise comparisons.

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2. Donut Chart: The Space-Saving Variation

A Donut Chart is essentially a pie chart with the center cut out.

• Primary Purpose: Same as the pie chart (showing part-to-whole relationships), but its design offers a specific advantage.

• Visual Encoding: Relies primarily on the arc length and area of the colored ring segments. The central hole eliminates the use of the angle at the center, theoretically shifting the viewer's focus slightly more to the arc length.

• Key Advantage: Information Density: The most significant advantage of the donut chart is the empty center. This space can be used to display summary information, such as the total value of the entire circle or a key metric related to the proportions being shown. This adds context without requiring additional visual space.

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 Alternatives for Proportions (When to Avoid Pie/Donut)

Because of the perceptual difficulty in judging angles and areas, analysts often prefer alternatives when precise comparison is necessary:

Alternative Chart	When to Use It	Encoding Used
Stacked Bar Chart	To compare proportions of several wholes simultaneously (e.g., market share composition over 5 different years).	Length (accurate)
Simple Bar Chart	When you want to compare the size of the parts directly (e.g., comparing the market share of Apple vs. Samsung).	Length (most accurate)

The Stacked Bar Graph: Composing the Whole

The Stacked Bar Graph is a powerful variation of the traditional bar chart, specifically designed to visualize the composition of a total value across different categories. It serves as an excellent alternative to pie or donut charts when you need to compare both the total size and the proportions of the parts.