Compare the scales
A line can look steep because its values change quickly, because the vertical axis is compressed, or because the chart is tall. Separate the data from the drawing.
Explain how units and axes shape the picture.
Start with the idea
An axis maps quantities to positions. On a linear axis, equal distances represent equal numerical differences. Moving from 100 to 110 takes the same space as moving from 200 to 210. Those changes are equal in units, but not in percentage terms.
Read the lowest and highest ticks. An axis running from 99 to 101 makes a change of one unit occupy half the plot. An axis running from 0 to 200 makes the same change look small. Neither scale changes the underlying value. A tightly cropped axis can reveal detail, provided its limits are clear.
Two charts with different vertical ranges are difficult to compare by eye. Put them on a shared scale when absolute values are the question. Use a common index when proportional movement is the question. Do not quietly switch units in the middle of an explanation.
A logarithmic axis uses equal distances for equal ratios rather than equal differences. This is useful in some contexts, but it needs a clear label. Our Compare Lab uses a linear scale in both modes, so a ten-point index change always takes the same vertical space.
A worked example
A moves from £100 to £110, while B moves from £200 to £210. Both gain £10. A gains 10%, while B gains 5%. The same absolute rise is not the same relative rise.
Your turn
A chart shows values from 98 to 102 but its vertical axis begins at 97. Does the large visual swing prove that the series doubled?
Open the exercise tool →Show the answer and reasoning+
No. The value changes by 4 units. Relative to 98, that is 4 ÷ 98 × 100 ≈ 4.08%. The cropped axis magnifies the visible movement without changing the calculation.
Read the scale, then describe the movement.
Related term
AxisSources & review
Original Charts Haven example. All stated numbers are illustrative and allow you to repeat the calculation.