A line rises by the same visual distance between 10 and 100 as it does between 100 and 1,000. The chart is not necessarily drawn incorrectly. A logarithmic axis can place equal ratios at equal distances, rather than equal numerical differences.
The Office for National Statistics guidance on axes emphasises clear scales and the extra difficulty of nonlinear axes. For readers, the first step is to inspect the tick labels before interpreting the shape of a line.
Compare multiplication with addition
On a base-ten logarithmic example, moving from 10 to 100 multiplies the value by ten. Moving from 100 to 1,000 also multiplies it by ten. The absolute increases, however, are 90 and 900. Equal vertical movement therefore does not mean an equal number of additional cases, customers, or units.
Write those two comparisons beside an unfamiliar chart. If the spacing makes sense as ratios but not as differences, look for a logarithmic-scale label. Do not assume that every logarithmic chart uses exactly the same displayed ticks or base.
A practical reading exercise is to choose two actual plotted values and calculate both the difference and the ratio. For example, a fictional series rising from 20 to 40 adds 20 and doubles. A second rise from 40 to 80 adds 40 and also doubles. The two statements answer different questions.
Before sharing the graphic, check whether the headline describes proportional growth or absolute change. Cropping away the axis can remove the information needed to tell them apart. If the scale is unclear, ask for the underlying values or a clearly labelled version. A dramatic-looking slope is not a substitute for knowing what a step on the axis represents.