Track 3 · Visualising and Exploring Data · Lesson 7 of 11
Line charts and time
Read trend, seasonality, one-off spikes and structural breaks from line charts, compare like with like over time, and display several series honestly.
A supply planner sees water-heater orders fall sharply one January and must decide: is this the usual post-winter dip, a bad month, or a permanent change? The line chart holds the answer — if it shows enough history and the planner compares the right months.
A line chart plots a numeric value against time and joins consecutive points, so the eye follows the sequence. Time goes on the horizontal axis, left to right, at evenly spaced intervals (a missing month should leave a gap, not be silently skipped).
Most business time series combine four ingredients:
- Trend: the long-run direction (growth, decline).
- Seasonality: a pattern that repeats with a fixed period (every 12 months, every 7 days).
- One-off events: holidays, promotions, outages — spikes or dips that do not persist.
- Noise: irregular variation left over.
A fifth, rarer feature matters most for planning: a structural break, where the level of the series shifts and stays shifted.
Included with a Formula Quest membership
It has 8 graded exercises with feedback, hints and worked solutions. After it you will be able to:
- Read trend, seasonality, one-off events and structural breaks from a line chart
- Compare like with like: year-over-year or whole-year changes for seasonal series
- Compute percentage changes with the right base
- Show several series with direct labels, small multiples or an index, not dual axes