Delivery Rates and Parcel Volume on One Board Slide
A logistics manager must show monthly on-time delivery rates for three depots over 24 months. Rates cluster between 88% and 98%, but one depot has a dip to 61% in a single month after a storm. She also wants to show total parcel volume, which ranges from 4,000 to 90,000 per month. Her current slide has a clustered column chart with all three depot rate series and a second series for volume on the same value axis. The rate columns are invisible, and volume dominates. The audience is a regional board that will see the slide for about 90 seconds. The board is non-technical and will ask about the storm.
1. What chart design would show the delivery rates and volume honestly in one glance, and what axis decisions are required?
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For a 90-second board glance, the chart needs one clear takeaway: rates were stable except the storm dip, while volume was much larger. A combo chart can work: show the three depot rates as lines on the primary axis, and total volume as light gray columns on a secondary axis placed behind the lines. Set the rate axis to about 50%-100%, label it On-time delivery (%), and annotate the 61% storm month. Set the volume axis from 0 to 100,000 and label it Parcels. Use a title that states the conclusion, and keep the legend to four items. If the board is easily confused, two aligned charts, rates above and volume below, are safer than one dual-axis chart.
2. Why is a secondary axis risky for this board, and how would you reduce the risk if you use one?
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A secondary axis is risky because two different scales can imply a relationship that is not there, and readers may compare column heights with line heights as if they share units. The volume columns could also dominate visually. To reduce the risk, the manager should color the axis labels to match their series, label both axes with units, mute the volume columns, and put data labels only on the storm dip and the latest month. She should avoid a title that claims causation between volume and delays. A short annotation such as Storm week: depot B fell to 61% helps. Testing the slide on one non-technical colleague is a good final check; if they misread it, split the chart.
3. The rate axis does not start at zero. What should the manager do, and when would a zero baseline be required?
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Line charts do not have to start at zero because they encode position, not length, but the truncated axis must be obvious. Starting the rate axis at 50% makes the 88%-98% band readable and still shows the 61% dip; the axis title and tick labels must make the scale clear, and the title should not exaggerate. If the manager switches to columns or bars for rates, the axis must start at zero because those marks encode value by length. Volume columns should always start at zero. A broken axis should be avoided unless it is clearly marked, because it can mislead.