Marks, channels and visual encoding
A chart maps each record or summary to a mark, and each field to a visual channel such as position, length or colour. Reading a chart starts with knowing what one mark means.
The lecture opens with one question: what does the chart make us see? A chart is a mapping from data to visual properties. A mark is the geometric object that represents a record or a summary: a point, a bar, a line or an area. A channel is a visual property of the mark that carries a value: position, length, angle, area, colour or shape.
W6 p. 3 turns one row into one mark. A part-time student with entry score 60 and final grade 70 becomes one square at x=60, y=70; the square shape encodes study mode. Reading the chart correctly therefore needs three answers: what one mark represents (here, one student), which field each channel carries, and in what units.
| Channel | Suits | Caution |
|---|---|---|
| Position on a common scale | Precise comparison of numerical values | Axis limits and units must be labelled |
| Length (bars) | Comparing amounts | The bar must start at zero |
| Angle and area (pies, bubbles) | Shares of one whole; rough magnitude | Harder to compare precisely; area grows with radius squared |
| Colour hue or shape | Separate identities (categories) | Needs a legend or label and must survive grayscale |
| Colour intensity | Ordered quantities, pale to strong | Weak for exact values |
Perception research summarised in the course references (Stanford CS448B, Harvard CS171) finds that readers judge position along a common scale and length more precisely than angle, area or colour intensity. Put the comparison that matters on the most precise channel. Every channel should carry a declared meaning: a decorative colour invites readers to search for a difference that is not in the data.
Source: W6 PDF pp. 1-37. Most numbers in the lecture are illustrative or constructed for teaching; pp. 26-27 reuse the Week 5 synthetic registration counts. The Week 6 lab manual (L6) practises Week 5 EDA reasoning and is worked through section by section in Lab work.
Practice problems
Supplementary study questions on the lecture concepts — not verbatim exam questions. Each answer counts toward spaced review.
1. Conceptual understanding
Definitions and distinctions.
In W6's one-student example, what does the square's shape encode?
Which channel lets readers compare numerical values most precisely?
2. Application
Apply the concept to a case or calculation.
In a bar chart of course counts, what is the mark?
In a scatter plot of entry score (x) and final grade (y), which channel carries final grade?
3. Interpretation and limitations
Evaluate assumptions and justify conclusions.
A scatter plot has one dot per registration, not per student. Why does that matter?
Bars are coloured randomly although colour encodes nothing. What is the risk?
Explain it in writing
Self-checked, not marked. Write a few sentences, then compare with the example answer. Your writing is saved as you type.
Show an example answer
Position along a common scale is judged most precisely, so readers can compare the scores accurately. Colour intensity gives only a rough sense of size.
Check: did you answer the question, use evidence or working, and avoid claiming more than the evidence supports?
Show an example answer
What one dot represents (for example one student, not one registration or one week), which variable each axis shows and in what units, and which population the 50 dots come from.
Check: did you answer the question, use evidence or working, and avoid claiming more than the evidence supports?