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Change over time

Line chart

Connects ordered measurements to show change, commonly over time.

  • Chart
  • Trend over time
  • Comparison

What it answers

How does the value change over time?

Shape
Connected points on axes
What it shows
Trend, peaks, troughs and gaps.

Example: Skye Ascent Ltd

The question

Will Skye Ascent Ltd’s cash stay above the board’s GBP 40 million floor?

Skye Ascent Ltd cash at year end, 2025 actual and May 2026 board budget

Skye Ascent Ltd cash at year end, 2025 actual and May 2026 board budgetSkye Ascent Ltd cash at year end in GBP million: 2025 actual 148.0; May 2026 board budget 2026 98.0, 2027 64.0, 2028 71.8. Board policy: never below 40. The budget assumes the planned launch schedule.Cash at year end, GBP million040801201602025 actual2026 plan2027 plan2028 plan148986471.8Board policy floor 40
Skye Ascent Ltd cash at year end, 2025 actual and May 2026 board budgetSkye Ascent Ltd cash at year end in GBP million: 2025 actual 148.0; May 2026 board budget 2026 98.0, 2027 64.0, 2028 71.8. Board policy: never below 40. The budget assumes the planned launch schedule.Cash at year end, GBP million040801201602025 actual2026 plan2027 plan2028 plan148986471.8Board policy floor 40
Skye Ascent Ltd cash at year end in GBP million: 2025 actual 148.0; May 2026 board budget 2026 98.0, 2027 64.0, 2028 71.8. Board policy: never below 40. The budget assumes the planned launch schedule.

Skye Ascent Ltd is a fictional company created for Insyt.

The misleading version

Same question, drawn badly

A line joining categories that have no order

A line joining categories that have no orderSkye Ascent Ltd 41%; other rocket builders 33%; universities 19%; industrial and other 7%.Share of revenue (%)0%10%20%30%40%50%Skye Ascent LtdOther rocket buildersUniversitiesIndustrial and other41%33%19%7%
A line joining categories that have no orderSkye Ascent Ltd 41%; other rocket builders 33%; universities 19%; industrial and other 7%.Share of revenue (%)0%10%20%30%40%50%SkyeOrbitalOther rocketbuildersUniversitiesIndustrial andother41%33%19%7%
Skye Ascent Ltd 41%; other rocket builders 33%; universities 19%; industrial and other 7%.

What goes wrong. A line suggests movement from one point to the next. Customer groups have no order, so there is no trend to show. Use a bar chart.

When to use it

Use it when

Ordered measurements and time trends.

Avoid it when

Unordered categories where joining values would imply a false sequence.

How to read it

Read time horizontally and value vertically; inspect direction and turning points.

Note

Numeric or date spacing matters when intervals are unequal.

The data you need

Layout
Long table
Fields
Time: date/datetime or ordered number; Value: number
One row is
One observation at a time or ordered position. What is the grain?
Rules
Parse dates consistently and sort ascending. Resolve duplicate timestamps by a declared rule. Missing values are not automatically zero.
Optional
Optional Series. Use regular spacing for methods requiring it.
Preparation
Parse and sort timestamps or sequence positions; aggregate duplicates deliberately and preserve gaps. Parse dates consistently and sort ascending. Resolve duplicate timestamps by a declared rule. Missing values are not automatically zero.
Units
Time uses a stated date/time unit and sampling period; the value axis uses the measure's unit. Rates need their own explicit denominator.
Example of the input layout: Long tableA long table: one row per observation, few columns. New data adds rows.TimeValueAABBC
A long table: one row per observation, few columns. New data adds rows.

Worked example

Data Jan 30, Feb 34, Mar 38. Connect in time order. The increase is 8 units from January to March.

TimeValue
2026-01-0130
2026-02-0134
2026-03-0138
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