Choosing a chart: bars, line or table

In the report, shortages by supplier are shown as a pie chart, weekly receiving time as bars and exact balances as a line chart. The reader spends time not on the data but on decoding the chart. Choosing a chart is not a matter of taste: it depends on the question the reader brings. When the question is clear, the type of chart almost chooses itself.

The question first

• How did it change over time? A line.
• Which category is bigger and which smaller? Bars, sorted from largest to smallest.
• What share of the whole belongs to what? With two or three parts a pie or one stacked bar, with more, plain bars.
• Are two metrics related? A scatter chart: each point is one item or one zone.
• Is an exact value needed? A table.

Excel, Google Sheets or another program often suggests a pie or three-dimensional chart. Do not rely on the ready-made choice; settle the question yourself first.

An example: shortages by supplier

Units short at receiving per 100 delivered: A — 0.8; B — 2.9; C — 1.4; D — 4.6; E — 0.5. The threshold is 2 units.

On a pie chart D gets 45 percent and B 28, but those shares are meaningless: shortage rates cannot be added up into a "whole" (4.6 + 2.9 + 1.4 + 0.8 + 0.5 = 10.2 means nothing). Lay out horizontal bars from largest to smallest: D, B, C, A, E, and draw a line at the threshold of 2 units. It is at once clear that two suppliers crossed it. The bars are horizontal because names are usually long (items or suppliers), so nobody has to read rotated text. The title writes itself: "Two suppliers exceeded the shortage threshold".

Bars start at zero

Stock accuracy is 96.4 and 97.1 percent. Start the axis at 96 and the bars come out 0.4 and 1.1 tall, so the second looks 1.1 ÷ 0.4 = 2.75 times higher. In fact the difference is 97.1 ÷ 96.4 ≈ 1.007 times, almost invisible. The length of a bar stands for the value, so do not cut off the zero line. On a line chart the axis may start above zero, because it shows the direction of change rather than height, but the axis values must be labelled.

Line and table

Time to receive one truck (minutes), 12 weeks: 38, 40, 37, 39, 41, 40, 44, 47, 49, 52, 51, 55. In a table you have to read this through, while on a line the rise is visible at once. The arithmetic confirms it: the average of the first six weeks is 235 ÷ 6 ≈ 39.2 minutes, of the next six 298 ÷ 6 ≈ 49.7. That is a rise of 10.5 minutes, about 27 percent.

On a line chart do not put more than 4-5 series, name them at the end of the line rather than in a legend, and make the most important one thicker. To show the weekly errors of eight zones, put not eight lines on one chart but eight small charts side by side with identical axes: each is clean and comparison is easy.

A table is the better choice when you need the exact figure (checking against 1C), there are no more than 10-12 rows, or the units differ. Align numbers to the right, keep the same number of decimals and use colour only for exceptions.

What to avoid

• Three-dimensional charts: the front bar looks bigger.
• Charts with two axes: by choosing the scales you can make the lines cross anywhere and invent a "relationship".
• Pie charts with many slices.
• A separate colour for every bar: one colour is enough; highlight only the ones that crossed the threshold.
• Everything on one chart: one chart should answer one question.

Conclusion

The question first, then the type of chart: a line for time, bars for comparison, a table for exact values. A final check: without reading the title, can you state the conclusion from the chart alone? If not, the chart is not answering the question. A chart shows reliable data well, but a wrong balance stays wrong even on a beautiful chart. CountLab breaks stock-count results down by category, zone and cause and presents them in the management report as a suitable chart or table.