A chart connects the totals of eight unrelated product categories with one line — what does that line claim?
answer
- what happens between two marks
- a line claims an interpolable path
- reorder the categories, watch the shape
- unordered means nothing lies in between
basics
~20 sA connecting line claims the space between two marks was travelled: that intermediate positions exist and the value passed through them. Between unordered categories nothing lies in between, so the line asserts a path that does not exist.
solid answer
~50 sA line is not merely a way of joining marks; it is an assertion about what happens **between** them. It says the horizontal positions are ordered, that positions between the drawn ones are meaningful, and that the quantity moved smoothly from one endpoint to the other at the rate the slope shows. That is exactly right for a dimension you can interpolate along — a timestamp, a dose, a distance — and false for unordered categories, where there is no point between *Footwear* and *Garden* and no fact makes one the predecessor of the other. The quick test is to reorder the categories: if the silhouette changes while every number stays identical, the silhouette was never in the data. Draw one bar or one point per category from a common baseline instead, so the reader compares magnitudes and reads no path.
go deeper
Recall the one sentence: a line asserts that the space between the marks exists and was travelled. Between unordered categories there is nothing in between, so use a mark with a shared baseline instead.
Explain the mechanism and the test. The line's slope is read before the labels are, the spacing was chosen rather than measured, and reordering the categories while the numbers stay fixed exposes a shape that came from the layout.
Show the judgment in review: name what the picture asserts, say which comparison the reader was actually asked to make, and give the alternative plus what it costs. Note that a tool rendering it silently is no evidence of correctness.
The angle worth owning is the standing rule: charts that leave your team assert only what the data supports. Decide whether that is enforced by review, by a house pattern for category comparisons, or not at all — and accept the cost of whichever you choose.
## What a connecting line asserts The **mark** is the thing actually drawn for each row or each group — a point, a bar, a line, a box, a tile. Most marks make a claim about one row at a time. A point says *an observation sat at this position*. A bar says *this group's quantity is this long, measured from a baseline every other bar shares*. A line mark is the odd one out: it is the only everyday mark that makes a claim about the space **between** two rows, and that claim has three parts. 1. The horizontal positions are **ordered** — one genuinely comes before the next, and the order is a property of the data rather than of the order the rows happened to arrive in. 2. Positions **between** the drawn ones exist and mean something, so a reader may put a finger halfway along and read a value off. 3. The quantity **passed through** those intermediate values on its way from one endpoint to the other, at the rate the slope shows. All three hold for a dimension you can interpolate along: a timestamp, a dose, a distance, a price point. None holds for eight unrelated product categories. There is no position between *Footwear* and *Garden*, nothing was measured there, and no fact of the world makes one the predecessor of the other. The line draws a path nobody travelled — and because the eye reads slope long before it reads tick text, that path is the first thing the reader takes away. ## The test that settles it in ten seconds Reorder the categories and redraw. **If the silhouette changes while every number stays identical, the silhouette was never in the data.** Alphabetical order gives one shape; largest-first gives a clean descent that reads as decline; the order the rows arrived in gives a third. An ordered dimension fails this test in the other direction: you cannot put March before January without lying about the data, and that is precisely why a line is allowed there. ## Ordered, unordered, and the case in between | the horizontal dimension | is there a meaningful point between two values? | what a connecting line then means | |---|---|---| | timestamp, dose, distance | yes, and the spacing is real | a rate of change the reader may interpolate | | an ordinal scale — small, medium, large | the order is real, the spacing is not | the direction is honest, the steepness is not | | unordered categories | no | a path that was never travelled | The middle row catches careful people. With an ordinal dimension the line is not lying about sequence, so nothing looks wrong — but the horizontal gaps were chosen, usually as equal steps, and the slope inherits that choice. Either say what the spacing means or drop the line. ## What to draw instead - **Bars from one common baseline**, one per category: length is read against a shared zero, the category positions carry no quantity at all, and reordering changes nothing a reader would take as a measurement. - **One point per category along a single line of positions**, when the labels are long or the categories are many — the same shared baseline, far less ink. - Choose the category order deliberately and make it legible, so the reader understands it as an editorial decision rather than mistaking it for something measured. - Keep the line for the moment the horizontal dimension becomes interpolable: the same categories measured every month, one line per category, each line tracing a single entity through time. There is one honest exception worth knowing. A chart that draws a line between two positions **to follow the same entity across two states** — the same team before and after, the same account at two dates — is using the line to carry identity rather than interpolation. The reader is meant to see which end connects to which, and the slope is a difference, not a rate. That reading works only because there is nothing in between to misread. ## Where the designs differ Charting interfaces do not agree about whether to stop you. Some treat a discrete horizontal dimension as genuinely discrete and will not connect marks until you state which rows belong to one series; the refusal is the design telling you the claim needs a justification. Others simply join the marks in the order the rows arrived and say nothing at all. **A chart that rendered without complaint is not a chart that was checked** — the absence of a warning tells you which tool you are holding, not whether the picture is true. ## What an interviewer is listening for Not "lines are for time series", which is a rule learned by rote and wrong at the edges. What earns the mark is naming the claim out loud — *a line asserts that the space between the marks exists and was travelled* — then applying it to the dimension actually in front of you, and finishing with what you would draw instead and why that alternative asserts less.
- When is a line between category positions legitimate?When the line carries identity rather than interpolation: the same entity drawn at two or more states — before and after, first visit and second — and joined so the reader can see which end belongs to which. The slope is then a difference, not a rate, and nothing lies between the endpoints to be misread. It is also fine once the horizontal dimension itself becomes interpolable, with one line per category across dates.
- The categories are ordinal — small, medium, large. Is a line honest then?Half honest. The order is a real property of the data, so the line is not inventing a sequence. But the spacing between the positions was chosen, almost always as equal steps, and the slope is read off that spacing — so the direction of the line is meaningful and its steepness is not. Say what the spacing represents, or use a mark that makes no claim about the gaps.
- Does sorting the categories largest-first fix the problem?No, it hides it. Sorting produces a smooth descent that looks more like a trend than the unsorted version did, so the false path becomes more persuasive rather than less. The order is worth choosing deliberately for readability, but the defect is the connecting line's claim about the space between marks, and no ordering removes that claim.
saying these in an interview costs you the question
- Says the line chart is fine because it looks cleaner
- Treats it as a style preference rather than a false claim
- Believes alphabetical order makes the dimension ordered
- Reads the slope between two categories as a rate
- Assumes the tool would refuse to draw an invalid line