A chart shows one coloured line per region; at seven regions the colours blur - what does one panel per region give instead?
answer
- hue is guessed at, position is read
- every hue costs a trip to the legend
- three or four categories, then stop
- identity becomes a label, not a colour
- between groups against within each group
basics
~20 sOne panel per region trades a hard perceptual comparison for an easy one: reading seven hues against each other becomes reading seven small charts side by side, each carrying its own label instead of a legend entry.
solid answer
~50 sColour hue is guessed at rather than measured, and every read costs a trip to the legend, so past three or four categories the mapping stops being held in the head. Drawing the same chart once per group, side by side, moves the group identity out of a hue and into two things read directly: a position on the page and a label on the panel. No group is then drawn over another, and each group's own shape becomes legible. What it costs is real: the comparison between groups used to happen at a shared position in one glance, and now it happens across space, which only works if the panels share their limits. So the question is not how many colours are left; it is whether the reader compares groups against each other or studies each group on its own.
go deeper
Recall that colour hue is a weak channel for many categories and that a panel per group replaces a legend lookup with a label read directly off the panel.
Explain what the split costs as well as what it buys: a one-glance comparison at a shared position becomes a comparison across space, and it only survives if the panels share their limits.
Choose by naming the comparison the reader will make, and account for the work a hand-built grid needs - equal limits, one legend, panel order, empty groups - that a declarative split derives.
The angle worth arguing is the standing rule: at what point does a team stop adding colours, and what must a panelled chart state about itself before anyone acts on it.
## The channel that runs out A **visual channel** is a property of a drawn mark - position along an axis, length, area, colour, shape - that a column's values can be carried by. **Binding** a column to a channel is the statement that this column's values are what that channel varies with. Channels are not interchangeable, because a reader recovers a number from each one with a different accuracy: position along a common axis is read most accurately, length nearly as well, and angle, area and colour hue are guessed at. Colour carrying an unordered category is not really read at all - it is *looked up*. The reader sees a hue, goes to the legend, finds the matching swatch, reads the name, and comes back. That cost is paid on every single read, and the number of hues a reader can hold apart is small: - two similar hues crossing in a dense part of the chart stop being separable at all; - the usable set shrinks for the substantial fraction of readers with a colour-vision difference; - it shrinks again on a projector, on a photocopy, or in a document somebody printed in grey. Three or four categories is the working ceiling most practitioners quote, and the important part is that the failure past it is **not aesthetic**. The reader does not decide the chart is ugly. The reader quietly stops telling the groups apart and reads the overall cloud instead, while believing the chart was understood. ## What a panel per group does **Panelling** - also called small multiples, or faceting - is the same chart drawn once for each distinct value of a grouping column, laid out side by side. The group identity stops being a hue to decode and becomes two things a reader gets for nothing: a **position on the page** and a **label attached to the panel**. What that buys: - each panel holds one group's marks, so overlap between groups disappears entirely; - the group name is read directly rather than matched through a legend; - a shape inside one group - a trend, a step change, a spike, a repeating wobble - becomes legible, because nothing else is drawn across it; - the set can be **scanned**: an eye sweeps a grid of small charts far faster than it untangles seven crossing lines. What it costs is the part people skip. The comparison between groups used to be made at a **shared position**: one glance, no memory. After the split it is a comparison **across space**, which asks the reader to carry one shape to another part of the page - and it only works at all if the panels are drawn against the same limits. ## Which layout answers which question | what the reader is actually asking | the layout that answers it | |---|---| | Which group is highest right now, and do the lines cross? | one set of axes, colour per group, only a few groups | | What does each group's own behaviour look like? | a panel per group | | How do the groups rank on one number? | neither - one mark per group against a common baseline | So the decision is not *the colours ran out, add panels*. It is: **name the comparison the reader will make, then choose the layout that makes that comparison cheap.** Running out of colours is usually a symptom that the comparison has moved from *between groups* to *within each group* - but not always, and two series that cross each other are still best overlaid. ## What the split costs you to build Here the designs in common use genuinely differ, and a sentence that is true of the one you learned first is false of the next one: 1. Where the panel split is **a clause of one chart specification**, adding it changes no other line. The scale is defined once for the whole chart, so every panel inherits the same limits; the panel labels and the single legend are derived; the layout is arranged for you. 2. Where you **lay out a grid of regions by hand** and draw one chart into each, you take on four jobs the first design did for you: keeping the limits equal so the panels stay comparable, producing one legend rather than one per region, deciding the order the panels appear in, and deciding what happens to a group with no rows. *It is one extra argument* is therefore accurate for some tools and a serious underestimate for others. Find out which you have before you promise the chart, because those four jobs are exactly the four places this kind of chart goes wrong. ## The decisions that remain open Choosing panels settles nothing by itself. Four things are still unchosen, and the reader is affected by all of them: whether the panels share one range or each fits its own, how many panels stay readable at the size the page allows, the order the panels are laid out in, and what a group with no rows looks like. Every one of those is a claim the picture makes whether or not anybody made it deliberately.
- When is one set of axes with a colour per group still the better choice?When the comparison is between the groups and there are only a few of them - two or three lines that cross tell their story at a shared position, in one glance. Split into panels, the same comparison forces the reader to carry one shape across the page from memory, which is slower and less accurate.
- What replaces the legend once the groups become panels?The panel's own label, sitting with the marks it describes, which removes the lookup entirely. Worth knowing how the legend arose in the first place: where colour is a mapped channel with a scale behind it, the legend is derived from the mapping and follows it; where colour is a constant passed per drawing call, the legend is written by hand and can quietly disagree with the picture.
saying these in an interview costs you the question
- Says more groups simply need a bigger or better palette.
- Treats panels as decoration rather than a change to what is compared.
- Claims panels are always clearer, even for two series that cross.
- Assumes colour and panels carry the between-group comparison equally well.
- Assumes the split is one setting in every tool, with nothing left to keep consistent.