# Principles of Graph Construction

1. Graphical Perception

2. Principles

# GRAPHICAL PERCEPTION

Goal: Communicate information

Author (you) -> graphic -> Reader

A graph is more effective than another if its quantitative information can be decoded more quickly or more easily by most observers.

Nancy Robbins, Creating More Effective Graphs

# Pattern perception depends on

Detection: recognition of geometry encoding values

Assembly: grouping of detected symbol elements

Estimation: assessment of relative magnitude of values

# A hierarchy of human graphical perception abilities

1. Position along a common scale (most accurate task)

2. Position along identical nonaligned scales

3. Length

4. Angle and slope

5. Area

6. Volume

7. Color: hue (red, green, blue, etc.), saturation (pale/deep), and lightness. Hue can give good discrimination but poor ordering.

# PRINCIPLES

1. Show the data

2. Avoid distorting the data

3. Aid comparisons

4. Maximise data:ink

5. Label data, minimize legends

6. Use clearly different symbols, colours

7. Consider 0

8. Dot plots are often better than bar charts

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## 8. Dot plots are often better than bar charts

Bar charts:

• High ink:data ratio

• Error bars cause perception errors

• Can only show one-sided confidence intervals well

• Thick bars reduce the number of categories that can be shown

• Labels on vertical bar charts are difficult to read