Bar Graphs

At a glance 10 rules

1. 1Read the scale firstNote the axis unit, the gridline step and where the axis starts; on an axis that does not start at zero, a bar twice as tall is not twice the value.
2. 2Stacked barsThe top of a stacked bar is the total; a segment is its top edge minus its bottom edge, so never read a middle segment's top as its value.
3. 3Unequal periodsWhen bars cover periods of different length (28, 29, 30 or 31 days; 4 or 5 weeks), divide each bar by its length before comparing.
4. 4Bars of growth ratesWhen bars show growth over the year before, a shorter bar above zero still means a rise; the value falls only in a year whose bar is below zero.
5. 5Readings are rangesA reading off a gridline is only near; for the largest possible growth take the new bar at its highest and the old bar at its lowest.
6. 6Average from a baseline$\bar{x} = b + \dfrac{\sum (x_i - b)}{n}$$x_i = \text{the } n \text{ bars},\ b = \text{a round level}$
7. 7Over the whole year$\dfrac{\sum \text{achieved}}{\sum \text{target}} \times 100$$\text{never average the percentages}$
8. 8More than 120%'X is more than 120% of Y' means $X - Y > \tfrac{Y}{5}$, or $5X > 6Y$; a tie is not 'more than'.
9. 9Share across yearsA segment's share rises only if it grows faster, in %, than the whole bar, which is the same as its ratio to the rest of the bar rising.
10. 10Share of a change$\text{contribution} = \dfrac{\Delta\,\text{segment}}{\Delta\,\text{total}} \times 100$$\Delta = \text{new} - \text{old};\ \text{can top } 100\%$