Comparing Two Bar Charts with Different Scales
A regional report shows two bar charts side by side. Chart A (Store North) plots monthly sales with a vertical axis from $0 to $100,000; the tallest bar (December) reaches $90,000. Chart B (Store South) plots the same metric with an axis from $0 to $200,000; its tallest bar (December) reaches $150,000. Both charts use the same bar width and are printed at the same physical size. A reader glances at them and says the two stores look 'about the same' because the bars reach similar heights on the page.
1. Why does the visual comparison mislead, and what are the actual December values?
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The axes have different maximums ($100,000 vs $200,000) but the charts are the same physical size, so a given bar height represents twice as much money in Chart B. December is $90,000 at North and $150,000 at South - South is 67% higher, not 'about the same.' The similar on-page bar heights come from the scale difference, not similar sales.
2. What change makes the comparison fair?
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Put both stores on a common vertical scale (e.g., $0–$200,000) and the same chart size, or plot both series as grouped bars on one chart. Only then does bar height map to the same value in both panels, letting a glance comparison be trustworthy.