Combined Charts

At a glance 9 rules

1. 1Find the common keyTwo charts always share a key (the same colleges, years or products). Turn each chart's figure for that key into one unit, usually a count, before combining.
2. 2Part from two charts$\text{rest} = \dfrac{\text{share}\ \%}{100} \times \text{total} - \text{known part}$$\text{unknown total } T\text{: each count} = \tfrac{\text{share}}{100} \times T$
3. 3Two pies, two totals$\text{value} = \dfrac{\text{share}\ \%}{100} \times \text{its own total}$$\text{compare values, not percents}$
4. 4Pie for one barWhen a pie splits one year's bar, its percents belong to that year only; reach any other year through the stated growth, not through the pie.
5. 5Rate from two charts$\text{average price} = \dfrac{\text{revenue (from the pie)}}{\text{units (from the bar)}}$
6. 6Weight by the 'per'A rate averages over the quantity after 'per': km per litre averages over litres, so overall mileage is total km ÷ total litres, not km shares times mileages.
7. 7TransfersWhen people move between groups, the grand total stays the same; track only the net gain or loss of the groups the question names.
8. 8Share after a change$\text{new share} = \dfrac{\text{group's new value}}{\text{old total} + \text{change}}$$\text{a loss is a negative change}$
9. 9Group % from two piesStaff pie and women pie: a department's women % is (its women share ÷ its staff share) × (all women ÷ all staff) × 100, so rank by the share ratio.