How a chart can make the same change look bigger

Two charts show the same monthly value rising from 98 to 102. In one the increase barely registers. In the other a line climbs across most of the frame. Before deciding which chart tells the better story, ask how it represents the numbers. Your eye reads the length of a bar differently from the position of a dot on a line.

With bars, check the starting point

Imagine two vertical bars for the values 98 and 102. If both start at zero, the second bar is about 4.1% longer. Start the bars at 95 and the visible lengths become three and seven units. The taller bar looks more than twice as high even though the underlying value increased by four. The Office for National Statistics recommends a zero baseline for bars so their lengths correspond to their values.

Check which axis measures value. If the bars run horizontally, the numerical baseline is often on the horizontal axis. For a chart with negative as well as positive values, zero is the reference from which the bars extend in opposite directions.

With a line, read the scale and the dates

A line chart links observations. A closer vertical scale can make a change visible when values cluster near 100. ONS allows that approach for a line chart when it helps show a trend. A line chart ranging from 95 to 105 might make those movements easy to see. The same line shown on a range of zero to 120 offers a different sense of the movement’s size. Read the labels before carrying that visual impression into your summary.

Look across the horizontal axis too. The steepness of a segment reflects both the numerical change and how much time the chart gives it. Compare the date labels before assuming that two segments represent equal intervals. Read a labelled gap as a gap in the observations.

Turn the picture back into a sentence

For our fictional chart, a useful description is “the measure rose by four units, from 98 to 102.” If you give the relative change, say it was about 4.1% from the starting value. That sentence works across either display and helps someone check what the image shows. If the numbers represent an index, carry the index label into your description.

• Find the unit and numerical range.
• Check the baseline on a bar chart and the date spacing on a line chart.
• Quote actual values when the slope or bar length is doing much of the persuading.