What it is
A line chart plots values against an ordered axis, usually time, and connects consecutive points. The slope of each segment shows how fast the value is rising or falling, so the shape of the line is the story: growth, decline, seasonality, a sudden break.
Several series can share the axes, one colour per series. Up to four or five lines stay readable; Chart Buffet labels them directly at their last points so you don’t have to look back and forth at a legend.
✓ Use it when
- Your first column is dates, times or years and you want to see the trend.
- You have many points — dozens or hundreds — where columns would become a thicket.
- You want to compare how a few series move relative to each other.
✗ Avoid it when
- The categories have no order (fruits, countries). Connecting them implies a progression that isn’t there — use bars.
- You have only two or three points — a short table or columns are clearer.
- The series are in different units. Never give them two y axes; plot them separately or index them to a common start.
Tips for a better chart
- The value axis of a line chart need not start at zero — the slope is the message — but say so if you cut it.
- Choose the chart’s shape so that the lines are mostly at about 45°; very flat or very steep lines hide changes (Cleveland, McGill & McGill 1988).
- Use smoothing only for decoration, not for sparse data: a curve between two points suggests values that were never measured.
- Highlight the series that matters in a strong colour and keep the rest quieter.
The maths
Between consecutive points (x1, y1) and (x2, y2) the line has slope (y2 − y1)/(x2 − x1): the average rate of change over that interval. A date axis spaces points by real elapsed time, so an irregular series still shows true rates.
Cleveland, McGill and McGill showed that slopes are judged most accurately when they centre on ±45°, which is why a chart’s aspect ratio matters — the idea is known as banking to 45° (Cleveland, McGill & McGill 1988).
On a logarithmic value axis equal vertical distances mean equal ratios, so steady percentage growth appears as a straight line.
A little history
Time-series line charts, too, begin with William Playfair. His 1786 Atlas plotted England’s imports and exports over eighty years as two lines, shading the gap between them as the balance of trade (Playfair 1786). Earlier scientists had plotted measurements against time, but Playfair turned the device into a way of arguing with data.
Through the nineteenth century line graphs became standard in science and economics, and the twentieth century’s statistical handbooks codified their rules. Tufte’s critique of “chartjunk” later pushed them towards the spare style common today (Tufte 1983).
Questions
What is the difference between a line chart and a line graph?
None; they are two names for the same chart.
Can I put two measures with different units on one line chart?
Chart Buffet deliberately doesn’t offer a second y axis: the alignment of two scales is arbitrary and invents correlations. Make two charts, or convert both to an index such as “100 = first year”.
References
- Playfair, W. (1786). The Commercial and Political Atlas. London: Debrett, Robinson and Sewell.
- Cleveland, W. S., McGill, M. E. & McGill, R. (1988). The shape parameter of a two-variable graph. Journal of the American Statistical Association 83(402), 289–300.
- Tufte, E. R. (1983). The Visual Display of Quantitative Information. Cheshire, CT: Graphics Press.