IS6335 Chap.4 Marks, Channels and Colour Design Rules
Marks, Channels and Colour Design Rules
What a mark is and what a channel is
A mark is the basic graphical element standing for an item: a point, a line, an area or a volume. A channel is a property of that mark you can vary to carry the value of an attribute. Week three is about the assignment between the two, and the published topic list for the week names marks, channels, and colour usage and design rules.
The week two encode material already landed the channel list: colour, subdivided into hue, saturation and luminance, then size, angle, curvature, shape, and motion with its own direction, rate and frequency.
What this week adds is that channels are not interchangeable. They differ in how accurately a reader decodes them, and the difference is a property of human perception rather than of taste.
Position on a common scale is decoded most accurately, which is why there are only two axes and why they are the scarcest resource in any design. Length and angle follow, then area and luminance, and hue, shape and motion carry the least precise readings of all.
Two properties decide whether a channel may carry an attribute
The first is whether the channel is ordered.
Hue is not: red is not more than blue, so hue can only carry a categorical attribute. Luminance and saturation are ordered, so they can carry an ordered one. The second is capacity, meaning how many distinguishable levels the channel offers.
Hue supports only a handful of categories before they stop being separable at a glance, which is why a chart with twenty coloured lines is unreadable however carefully the palette was chosen.
Two failures account for most bad charts and both follow from ignoring those properties. Giving an ordered attribute to hue leaves the reader unable to tell which value is larger without consulting a legend repeatedly.
Giving a categorical attribute to a continuous channel such as luminance invites the reader to infer an order that does not exist. Both render perfectly and both say something false.
A budget, spent in a fixed order
Because channels differ in accuracy, the assignment is a budget.
Rank the attributes by the task first: the one the reader's question actually turns on, not the one with the most interesting name, gets the strongest channel. Assign position next, because there are only two axes. Work the remaining ordered attributes down the accuracy ordering, accepting that precision falls at each step and saying so in the write-up.
Give categorical attributes to hue or shape, counting the categories first and grouping or faceting if there are more than about seven. Then check what is left unencoded, because every attribute you did not assign is a decision and a reader who needs it will not find it.
Colour deserves separate handling because treating it as one thing causes most colour problems. Hue is categorical and low capacity.
Saturation is ordered, low capacity, and interacts with mark size, so the same value reads differently on a thin line and on a large area. Luminance is ordered and is what survives greyscale printing, photocopying and the most common forms of colour vision deficiency, which makes it the safe ordered channel whenever a design has to work in more than one medium.
Ordering direction then decides the shape of the ramp: single ended for sequential data, neutral at the meaningful zero for diverging data, and returning to its start for cyclic data.
Arranging space once the channels are chosen
The encode material pairs the channel list with a second list about space: express, separate, order, align and use. Expressing shows values as they arrive.
Separating splits them by a category. Aligning gives every group a shared baseline. Ordering sorts them so that rank becomes readable directly.
Aligning and ordering are the two that repay the least effort. Groups sharing a baseline can be compared by position, the strongest channel, while groups that do not have to be compared by length, which is weaker.
An alphabetical axis hides rank, so if the reader's question is which category is largest, sorting by value answers it without a single number being read. Neither move changes a value, and both change how much work the reader has to do, which is the whole subject of the week.
What this chapter covers
- 01
Marks as points, lines, areas and volumes
- 02
The channel list, and why the order of accuracy matters
- 03
Ordered against unordered channels, and channel capacity
- 04
The channel budget, spent strongest first
- 05
Hue, saturation and luminance as three different channels
- 06
Ramp shape for sequential, diverging and cyclic attributes
- 07
Express, separate, order and align
Repair a chart that is correct and unreadable
- 2Name the attribute types in play.
- 2Diagnose the quantitative attribute's channel.
- 2Diagnose the categorical attribute's channel.
- 2Propose a replacement and name its cost.
Key terms
- Mark
- The basic graphical element standing for an item, such as a point, a line or an area. The choice of mark fixes which channels are then available.
- Channel capacity
- How many distinguishable levels a channel can carry. Hue runs out after a handful of categories, which is why a twenty-line chart cannot be rescued by a better palette.
- Common baseline
- A shared starting line that lets marks be compared by position rather than by length. Aligning groups onto one is the cheapest accuracy gain available in a bar chart.
- Luminance
- The lightness channel of colour. It is ordered and it survives greyscale printing and the common forms of colour vision deficiency, which makes it the safe carrier for an ordered attribute.
- Colour ramp
- The sequence of colours mapped onto an ordered attribute. Its shape has to match the attribute's ordering direction, with a neutral midpoint fixed at the meaningful zero for diverging data.
Marks, Channels and Colour Design Rules FAQ
Is a rainbow colour scale really that bad?
For an ordered attribute, yes, and the reason is structural rather than a matter of fashion. Distinctness is not the requirement; order is. Hue is an unordered channel, so a reader cannot tell which of two hues is higher without consulting the legend every time, and a rainbow ramp also has uneven perceptual steps, so equal differences in the data look unequal on the page. It then collapses entirely in greyscale.
A single-hue luminance ramp keeps the order visible, keeps the steps even and survives printing. For a genuinely categorical attribute with few levels, distinct hues are exactly right.
How many categories can I colour before it breaks?
Fewer than you will want. Beyond roughly seven distinguishable hues readers stop identifying categories by colour and start hunting the legend, at which point the encoding has failed even though the chart is correct. Three remedies are available and each is a design decision to state. Group the small categories into a single other bucket. Move the category to position by faceting into small multiples.
Or, when the question is about one category, highlight that one against a neutral background and let the rest be context. Adding a twelfth colour is not among the remedies.
Assessment move
Take one figure a week from your own project and write the channel assignment out as a list: attribute, type, channel, and the task it serves. Then delete a channel and see whether the figure still answers the question. If it does, the channel was decoration and the design has room for something the reader actually needs.
This exercise is also the fastest way to produce the justification sentences the individual report is marked on.
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