These two message types get their own forms because a general comparison rule sends you to the wrong chart for both.
How do you chart a ranking?
A ranking message is about order. Top five suppliers, bottom quartile of branches, the position a fund holds among its peers. The level may be almost irrelevant, and what the reader must be able to do is read positions off quickly and see gaps between neighbours. A sorted dot plot on a common scale does this better than bars. With bars the eye goes to the total length of each bar and misses the differences between the tips, and at fifteen or more categories the bar field turns into a wall of ink.
Rank change over two periods is the slope chart again. It's one of the few places where crossing lines tell you something. Usually they're noise. If eight categories are ranked in two periods and three of them swap places, the crossings are the message.
Advertisement
How does a diverging bar chart show a deviation?
A deviation message is about the gap from a reference: budget, prior year, benchmark, target. The natural form is a diverging bar centred on zero, sorted by the deviation, with the positive and negative directions distinguished. A cost variance table makes the shape obvious. Six budget lines totalling 325.0, actuals totalling 332.0, an overall variance of 7.0 or 2.15%. Raw material is 13.60 over, freight 5.80 over, overheads 1.90 over, labour 1.60 under, power 0.90 under and marketing 11.80 under. The diverging bar puts the three overruns on one side and the three savings on the other, sorted, and your reader gets the composition of a small net number in a second.
That's when the sort question gets sharp. Sorted by absolute variance, raw material is the top row at 13.60. Sorted by percentage of budget, marketing is the top row at 24.58% under. Both sorts are fair. They produce different top rows, and the top row is what a reader remembers. Choose the sort that matches your sentence and say which one you chose.
Should a variance chart show percentages or money?
Deviation is also where relative and absolute stop being interchangeable. Freight is 17.06% over budget and 5.80 in money. Raw material is 10.63% over and 13.60 in money. If the message is about control, the percentage is the point. If the message is about cash, the money is the point. A chart that shows one while the title claims the other is a defect the checker won't catch, because both numbers are true.