Change over time
Horizon chart
Divides an area chart into value bands and overlays them to compress its height.
- Chart
- Trend over time
- Deviation
What it answers
Where are large excursions in a compact time series?
- Shape
- Folded and overlaid colour bands
- What it shows
- Timing and magnitude bands of deviations.
Example
The question
Which servers ran hottest, and when, over the last day?
CPU load above and below normal, four servers (horizon chart)
Illustrative data. It belongs to no company.
When to use it
Use it when
Dense aligned time-series displays for experienced readers.
Avoid it when
Unfamiliar audiences without a clear key or when exact values dominate.
How to read it
Read colour bands using their thresholds and signs; compare time positions.
Note
Define band thresholds and negative-value handling.
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
- Choose value-band width and a reference; split signed excursions into bands before folding/overlaying.
- 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.
Worked example
Data deviations 2,7,-3 with band width 5. Value 7 occupies the second positive band; -3 uses the negative encoding.
| Time | Value |
|---|---|
| 2026-01-01 | 30 |
| 2026-02-01 | 34 |
| 2026-03-01 | 38 |