Data storytelling: one chart, one point
A report full of charts often says nothing. How to make each chart say one thing: start with the sentence, show the difference, use color as a highlighter.
You know the report. Twenty charts on one screen, each neatly formatted, everything reconciles with the source. Everyone looks at it, nods, and after the meeting nobody knows what to do with it.
The problem is not that the numbers are wrong. The problem is that the report claims nothing. It shows, and leaves the reader to work out what there is to see. The reader does not do that.
Data storytelling sounds like marketing. It is really discipline: deciding in advance what the reader has to see, and leaving out everything that does not help.
In the article on the data model I wrote that the charts are the least work. That still holds. But it is also the place where a good model loses its story.
Write the sentence first
Before you pick a chart, write in one sentence what it has to say. Not "costs per category", but "costs are 0.4 percent over forecast, mainly because of payroll and transport".
If you cannot write that sentence, you do not know yet what you want to show. No chart type is going to solve that for you.
Then put that sentence above the chart, as its title. A title that names the subject makes the reader search. A title that names the conclusion lets them check. The second is much faster.
Show the difference, not the totals
In a cost report I built as an example, actual cost up to September came to 1,335,800 euros, against 1,330,000 forecast. A difference of 5,800 euros.
Put those two totals side by side on an axis that starts at zero and you see two bars of the same height. Cut off the axis to show the difference and 5,800 euros looks like a mountain. Both tell the wrong story.
The answer is to show the variance itself, line by line, around a zero line. Then you see that payroll and transport together ran 32,300 euros over forecast, and that lower purchasing made up most of it. That is the story. The totals were only the occasion.
Make the comparison easy
Every chart is a comparison. Pick the shape in which the eye makes that comparison most easily.
Eyes compare lengths well and areas badly. Two overlapping circles of 36 and 57 percent look nice and are unreadable. Two segments of one stacked bar say the same thing, and you see the difference at once.
The same goes for small line charts in a table. Scale each one to its own minimum and maximum and every row looks equally lively, or equally flat. Then you are no longer comparing anything. Scale on something that is comparable across the rows, or show a different measure.
Color is a highlighter
If everything is colored, nothing stands out. Put what does not matter in gray and give one color to what the reader has to see.
And never let color be the only carrier. About one in twelve men has trouble telling red from green. An arrow up or down next to the variance costs nothing and works for everyone.
Explain the dip before anyone asks
In the same example report, the ratio of cost to revenue drops sharply in July. That is not an error. It is the construction holiday: little revenue against the same payroll.
Anyone who does not know that sees a problem. A short note on the chart prevents that discussion. The best report answers the question before the reader asks it.
From summary to detail
You read a page from top to bottom. Build it that way too.
At the top, the few numbers that say how things are going. Below that, the chart that says why. At the bottom, the detail for anyone who wants to check. Whoever stops after the first row has seen what matters most. Whoever reads on finds the evidence.
Title and data have to say the same thing
Small, but it undermines everything. If the title says budget and the table says forecast, the reader wonders which of the two they are looking at. They are two different things, as your ERP does not store a budget explains, and one doubt is enough to stop believing the rest of the report.
The claim
A report that only shows leaves the work to the reader, and the reader does not do it. Decide what you want to say, put it in the title, and build every chart so the eye makes that one comparison on its own.