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Data Visualization

Also: data viz, charting, information visualisation

Data visualisation is the practice of encoding numbers as position, length and colour so that a comparison a reader would otherwise have to calculate becomes something they can simply see.

Assessment. A chart title should state the finding, not label the contents: Renewals fell in every region except the Nordics is a title, Renewals by region is a filing label. A reader left to work out what you meant will reliably supply a meaning of their own, and it will be the flattering one.

Pick the encoding before the tool

Why some charts work and others fail is not a question of taste. People judge position along a common scale accurately, length almost as well, and angle, area and colour intensity progressively worse. Every chart type is a wager on one of those channels. A pie chart wagers on angle, asking a reader to compare wedges separated by a few degrees, which is why it fails as soon as there are more than a handful of categories or any two sit close together. The replacement is not a prettier pie. It is a bar chart laid out horizontally and sorted by value, which uses length instead of angle, gives long category names room to be legible, and puts the ranking into the order of the rows where it can be read at a glance. The same reasoning disposes of most of the gallery: donuts, radar plots, anything rendered in three dimensions, and the word cloud, which encodes frequency as area and is decoration wearing the clothes of evidence.

  • Comparing categories: horizontal bars, sorted by value rather than alphabetically, unless the categories carry an order of their own such as age bands.
  • Change over time: a line, with time running left to right. Begin the axis at zero when the quantity has a meaningful zero; when it does not, truncate openly and say so on the chart.
  • Relationship between two measures: a scatter plot. Add a fitted line only if you can defend the model behind it.
  • Distribution: a histogram, or a box plot when several groups are being compared. A chart showing only averages has hidden the spread that the argument probably turns on.
  • Composition: bars again, or a stacked bar when there are two or three parts and the total matters. Stacked areas with many series are unreadable above the bottom band, because only that band has a flat baseline to measure from.
  • Too many series to hold in the head: small multiples. A grid of identical little charts is better than one crowded chart, and better than a second vertical axis every time.

That leaves the question glossaries dodge by retreating into principles: what to learn. Split it by audience. For charts you are drawing in order to think, use Matplotlib inside the notebook where the data already sits, accept that the defaults are plain, and refuse to spend an afternoon styling something you will delete before lunch. For charts other people will open by themselves, learn one business intelligence product properly rather than three of them badly: Power BI if the organisations you are aiming at run on Microsoft, Tableau otherwise, and the second takes about a week once the first is fluent. D3 is not a charting library and should never be chosen as one. It is a programming project for building a bespoke interactive graphic, appropriate when a newsroom would reach for it and almost never when a company reports on itself. Underneath all three sits the same craft: subtraction. Most charts improve when the gridlines fade, the legend is replaced by labels sitting on the lines themselves, and the second idea moves to a chart of its own. A plain graphic with an unbroken axis and a title stating what happened outdoes an elaborate one in any tool you care to name.

In practice

The chart to stop drawing is the dual axis: two series, one scale on the left, a different one on the right, usually revenue set against something like headcount or ad spend. It is popular because the two lines can be made to converge or cross, and it is worthless for precisely that reason, since the apparent relationship is manufactured by whoever chose the two ranges. Nudge either axis and the story changes obediently. Two honest replacements exist. Index both series to a shared starting point and plot them on one axis, which makes the comparison about relative movement and admits as much; or stack two charts sharing an x axis, so the reader compares them vertically without being told a relationship is there. If the relationship is the point, plot one measure against the other as a scatter and let the reader see for themselves how tight it is.

Often confused with

Data Analysis
Analysis produces the finding; visualisation is a decision about how a reader receives it. Both can fail independently, and a beautiful chart carrying a wrong conclusion is the most expensive combination.

Key takeaways

  • →Chart choice is a claim about human perception: position and length are read accurately, angle and area are not, which rules out the pie and most of its relatives.
  • →Choose the tool by audience, not by fashion: Matplotlib to think with, one BI product to deliver with, D3 only for a bespoke interactive piece.
  • →Almost every draft improves by subtraction, and the title should carry the finding rather than name the axes.

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FAQ

Is it ever acceptable to truncate the vertical axis?
Yes, on two conditions. The quantity must have no meaningful zero, which is true of temperatures, index values, ratings and share prices, and the truncation must be visible rather than sneaked in, through an obvious axis label or a break marker. On counts and revenue, where zero means something, starting elsewhere exaggerates a movement and a reader is entitled to call it a trick.
How should colour be used?
Give colour exactly one job per chart and let position or length carry everything else. Distinct hues for categories, a single hue ramping in lightness for ordered quantities, and two hues meeting at a neutral midpoint when a value can be above or below a reference. Check the result against red-green colour blindness, and never use a rainbow ramp for a continuous measure, because it invents boundaries where the data has none.
How many charts belong on one dashboard?
Fewer than you have. A screen should answer one question and prompt one decision. If a reader has to scroll to find the important panel, the prioritisation was never done and has instead been passed to whoever opens the file, every morning, forever.
Do I need design skill to be good at this?
Not in the sense of drawing ability. The useful skills are measurement and restraint: knowing which comparison the audience needs, choosing the encoding that renders it accurately, and deleting everything that competes with it. Typography and spacing matter at the margin, and one chart that is correct and ugly outranks three that are elegant and misleading.

Sources

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Last reviewed 26 September 2026 · Getting Digital