Line charts are the default language of time in consulting. When you want to show how something evolves—grows, falls, cycles, spikes—lines are usually the first tool you should consider. They are compact, intuitive, and powerful at revealing patterns that tables and even columns tend to hide.
Used well, line charts show direction, pace, and volatility at a glance. Used poorly, they become a tangle of “spaghetti” where nothing is legible and nobody remembers the point. This chapter is about staying on the right side of that line.
6.1 When to use line charts (and when not to)
Use a line chart when your horizontal axis is time or another truly ordered, continuous dimension (e.g., test cycles, project phases that behave like time) and the story is about how a quantity evolves across that dimension.
Line charts are particularly effective when:
- You care about shape: steady growth, plateau, decline, or step change.
- You want to see volatility or seasonality: periodic peaks and troughs.
- You need to compare trajectories, not just levels, across a small number of series.
Consider not using a line chart when:
- The x-axis is categorical (e.g., business units, products) rather than ordered. A line connecting unrelated categories implies a continuity that is not there; bars are better.
- You have only two periods. Two points do form a line, but the visual gain over a simple two-column chart is minimal.
- You are primarily discussing composition (part-to-whole) rather than a single metric; stacked columns or 100% stacks usually do a better job.
- You need to compare many categories across one period; that’s a bar chart problem, not a line chart one.
The core test: if your language is “over time,” “trajectory,” “evolution,” or “pattern,” lines are your friend. If your language is “by x,” “across y,” “share of,” or “breakdown of,” you probably want something else.
6.2 Simple “data over time” line chart
A simple single-series line chart is the purest form: one metric, one timeline, one story. Think revenue over months, utilization over weeks, churn over quarters, market share over years.
Design principles for a clean single line:
- Put time on the horizontal axis, left to right, with evenly spaced intervals. Avoid irregular spacing unless you clearly signal it.
- Choose a reasonable time window. Too short and you risk overfitting noise; too long and relevant changes get visually compressed.
- Decide whether the audience needs exact values or primarily the pattern.
- If exact values matter, label key points (start, end, peaks, troughs).
- If the pattern is the star, keep labels light and let the shape speak.
- For the vertical axis, you do not always need zero. For line charts it is acceptable to start at a non-zero baseline to show meaningful variation, but:
- Make the range explicit.
- Avoid extreme zoom that turns minor fluctuations into what looks like a crisis.
In many decks, a strong simple line chart with an assertive title is all you need: “Market share has recovered steadily since the 2021 shock.” You do not impress anyone by adding two more series that dilute that message.
6.3 Multiple line chart (comparing series over time)
Multiple line charts show several series on the same timeline. This is where line charts become powerful—and where they are most often abused.
Good use cases:
- Comparing client vs main competitors over time on the same metric.
- Showing regions or segments on a shared scale: which one is growing fastest.
- Comparing scenarios: Base case vs Upside vs Downside.
To keep multiple lines usable:
- Aim for no more than three or four lines on a main-page chart. Beyond that, you quickly move into “spaghetti” territory.
- Use a dominant highlight line and make the rest quieter. For example, the client in a strong color and competitors in muted grays. The reader’s eye should land on the hero line first.
- Label lines directly at their ends when possible instead of relying only on a legend. That removes the back-and-forth eye movement that costs time and attention.
- Ensure all lines share the same units and scale. If you catch yourself wanting dual axes just to get everything on one chart, stop and reconsider; it usually confuses more than it helps.
If the chart starts to feel dense, consider alternatives: a panel of small multiples (one line per region) or focusing on the most important two lines in the main story and leaving the rest in an appendix.
6.4 Growth curve and evolution line charts
Not all lines are raw levels. In consulting, we often need to show relative growth or evolution from a common starting point. For that, indexed or growth-curve line charts are essential.
In an indexed line chart, series are rebased to a common starting value (often 100 in a base year) to show relative change, not absolute size. This is useful when:
- Entities differ greatly in initial scale (e.g., large vs small markets).
- The question is “Who has grown fastest since 2020?” rather than “Who is largest now?”
Key practices:
- Make the indexing explicit in the title or axis (“Indexed to 100 in 2020”).
- Use indexing only when the rate of change is truly the point. If absolute gaps matter more, stick to raw values.
- Be careful when mixing indexed charts and absolute charts in the same section; clearly signpost which one the reader is looking at.
“Growth curve” or “evolution” lines also show:
- Cumulative metrics (e.g., cumulative savings, cumulative sign-ups) vs period flows. Clarify which you are using; cumulative lines will never go down unless there are negative values.
- S-curve patterns (e.g., adoption of a new product, ramp-up of a program). When you see an S-curve, consider whether labelling the phases (early, rapid growth, saturation) would help the reader understand where they are on the journey.
Indexed and evolution lines help shift the conversation from absolute size (“We’re smaller”) to trajectory (“We’re growing faster and will catch up in three years if this continues”).
6.5 Peak and arrow line charts (highlighting key points and inflections)
Sometimes the story is not the entire line, but specific moments along it: a sudden spike, a structural break, a clear inflection. In these cases, you can treat the line as context and design the chart to pull the eye to 1–2 critical points.
Typical uses:
- A peak in complaints or downtime tied to a known event.
- An inflection point where growth accelerates or decelerates after a decision.
- A local minimum or maximum that sets up your recommendation (“we need to get back to here”).
Design techniques that work well:
- Use a small circle or marker on the key point, combined with a short annotation (“New pricing launched,” “System migration”).
- Add a subtle arrow pointing to the inflection or peak, with a short explanation of its significance.
- Consider a faint vertical reference line at the critical date to support “before/after” framing.
Keep these elements sparse. One or two highlighted points are powerful; ten are noise. Your title should echo what you are highlighting, e.g., “Service levels stabilized quickly after the Q2 intervention.”
6.6 Radial line chart (when circular timelines are appropriate)
Radial (or circular) line charts plot a line around a circle instead of along a straight horizontal axis. They are niche tools in consulting, but they have a legitimate use: repeating cycles over a fixed period.
Consider a radial line when:
- The story is about within-period patterns, e.g., hourly demand over a day, daily volume over a week, monthly sales over a year.
- You want to emphasize seasonality or cycle rather than long-term trend.
- The audience is comfortable with non-standard visuals and you have time to explain the form once.
They can work well for:
- Showing time-of-day peaks in call volumes or web traffic.
- Illustrating seasonal spikes in demand across a calendar year.
Use them with caution:
- Do not use radial lines to show long, non-repeating timelines; a normal line chart is almost always clearer.
- Be explicit in labeling the cycle (e.g., months around the clock face) and consider marking major points (Q1, Q2, etc.).
- Limit the number of overlaid series—ideally one, sometimes two. More than that and the circular format quickly becomes cluttered.
Radial charts are best reserved for moments where the cyclical nature of the data is itself the story. Otherwise, the simpler rectangular line wins.
6.7 Common pitfalls and good practices for line charts
Line charts look simple, which is why so many flawed ones survive on client decks. A few recurring issues are worth watching for.
Common pitfalls:
- Too many lines: a half-dozen series with similar colors and many crossings. If you cannot describe the chart in one breath, it is probably overloaded.
- Dual y-axes used to cram unrelated scales together (e.g., revenue in $M and margin in % on the same chart). This almost always confuses. Prefer two charts or a small, clearly explained combination chart.
- Over-zoomed axes that dramatize minor fluctuations or under-zoomed axes that flatten real swings.
- Irregular time intervals shown as if they were regular (e.g., 2019, 2020, 2022), which distorts perceived slopes. If intervals are irregular, label them clearly and reconsider whether a line is appropriate.
- Excessive data markers (dots on every point) that create visual noise. Markers should be used to highlight, not to decorate every observation.
A quick checklist before you send a line chart to a client:
- Is the question clearly about evolution over time (or another ordered dimension)?
- Are there no more lines than absolutely needed for the story?
- Are the scales honest and consistent across related slides?
- Is the hero line visually clear, and does the title state the main message as a sentence?
- Are key events or inflections highlighted simply, if they matter to the argument?
If you can answer yes to those questions, your line charts will do what they are meant to do in a McKinsey deck: make complex temporal patterns instantly legible to a very busy executive, and support the narrative you are trying to drive.
