This entry is part of the Chart Index, the reference library for the Chart Design Field Guide.

The slope chart is a line chart with exactly two time points. That constraint is its strength. With only a before and after, the eye reads the line as a single piece of information: direction, magnitude, and rank change, all in one glyph. The chart trades the granularity of a multi-point line for a clarity of comparison that no other form offers.

Tufte calls this a small noncomparative, highly labelled display. Each line in a slope chart is a single category compared between two states, and the slope itself — flat, climbing, falling, crossing — is the message.

What it is

A slope chart maps two time points (or two states) to the left and right edges of the chart, and a quantitative variable to vertical position. Each category is rendered as a line connecting its starting value (left) to its ending value (right). The slope of each line encodes the rate of change; the crossing of lines encodes rank changes.

The hero chart above compares six customer segments before and after a platform relaunch. Four lines climb, one is flat, one falls — and that is legible before a single number is read. The falling line is the story: education customers were the most satisfied segment before the launch and are in the middle of the pack after it, and the crossings show exactly which segments overtook them. Every endpoint is labelled with its value, so the chart needs no y-axis. Randomise the scores and watch the lines re-sort at the right edge; the labels nudge apart when two would collide.

When to use it

Slope charts are the right choice when:

  • You have exactly two time points (or two comparable states): before/after, pre/post, last year / this year.
  • The reader's question is "who moved up, who moved down, by how much?"
  • You have 5–15 categories — enough that ranking matters, few enough that the lines are distinguishable.
  • The direction of change matters more than absolute values.
  • You want to show rank crossings — which lines pass which.

When not to use it

  • Three or more time points. A slope chart with three points is just a line chart. Use a line chart.
  • Many categories. Twenty crossing lines becomes a tangled web. Use small multiples or a heatmap of changes.
  • Comparing magnitudes precisely. Slope encodes change, not absolute value. For magnitude comparison, a bar chart is more accurate.
  • No category-level identity. If the categories are not stable (different categories pre and post), the connecting line is misleading.

Design principles

Show the change as a slope, not as two bars

The conventional way to draw a before-and-after is a grouped bar with two bars per category. The pair below holds the hero's six segments both ways.

As a grouped bar: direction hidden
Two bars per segment. To find out whether education fell, the reader compares the heights of the fourth pair and subtracts; to find out who fell furthest, they do that six times. Nothing in the picture points down.
As a slope chart: direction is the picture
The same twelve numbers. One line slopes down, and the eye finds it instantly; its steepness says how far, and the three lines it crosses say who overtook it. The bar chart asked for arithmetic; the slope chart asks for a glance.

Direct-label every endpoint

Every line ends on the right; every label sits next to its endpoint. Reading order: eye lands on right endpoint, picks up the label and value, traces back to the left endpoint. No legend, no axis, no chartjunk.

Use consistent axis range across both ends

Both the left and right edges share the same value scale. The slope is meaningful only if the y-axis is uniform across the chart. A common error: scaling each side independently, which fakes the changes.

Highlight the lines with a story

If one or two lines carry the message (a segment that dropped, a segment that crossed), give them the accent colour and weight. Other lines fade to a neutral grey. The hero lines pull the eye; the context lines provide the rank baseline.

One segment highlighted
Education in red, everything else in hairline grey. The chart now makes a single claim — this segment fell while the rest rose — and the grey lines remain as the evidence for the second half of the sentence.

Strip the y-axis

Tufte's slope charts have no y-axis. The endpoints are labelled with their values, the slope is the encoding, and an axis would only add ink without adding information. Let the labels carry the precision.

Anchor at zero only if proportional change matters

Like a line chart, the y-axis of a slope chart usually starts above zero — the data range, padded modestly. The exception is when proportional change is the message; then anchor at zero.

Keep the line count down, or highlight

Beyond a dozen or so categories the lines begin to tangle and the labels to stack. Break a long list into themed small multiples — one panel per region, per segment, per cohort — or keep one panel and let colour do the editing.

Fourteen lines, all in colour
Share of trips by mode for fourteen regions. Every line is legible on its own, but with the whole set coloured the crossings in the middle are a tangle and the reader has nowhere to start.
The same fourteen lines, two highlighted
Canberra and Sydney's inner suburbs carry the story of the decade; the other twelve are context in grey. Nothing has been removed, but the chart now has a first sentence and a second sentence instead of fourteen simultaneous ones.

Annotate the rank crossings

When two lines cross, that crossing is often the story. Mark it with a small note, or use line weight to draw attention. Education fell below Enterprise post-launch — a sentence-long annotation explains what the chart shows.

Anatomy

The composition of a slope chart
An anatomical guide

The slope chart is structurally minimal: two vertical positions, a set of lines with endpoint dots, and direct labels at each end. Tufte's preferred form for two-point comparison.

  • Line chart — for three or more time points. Slope charts are a degenerate case.
  • Grouped bar — the conventional alternative. Encodes the same data but hides direction.
  • Dot plot — two dots per row joined by a connector: a horizontal slope chart, useful for many categories.
  • Bump chart — slope chart for ranks, with multiple time points.
  • Connected scatter — points in 2D space connected in order. Same idea, two variables instead of one over time.

Reading list

  • Tufte, E. (1983). The Visual Display of Quantitative Information. The original treatment of the slope graph.
  • Few, S. (2012). Show Me the Numbers. The case for slope charts in business communication.
  • Cleveland, W. & McGill, R. (1984). Graphical Perception. Why slope is read pre-cognitively.