The case in three lines. The Green Gazette's front page says LIBRARY VISITS PLUNGE over a bar chart in which this year's bar is a quarter the height of last year's. The numbers are right, 1,180 and then 1,120, but the chart's axis starts at 1,100, and the school has fifty fewer pupils. The librarian may lose his Thursday evenings, a Year 7 girl has returned her books, and Tara has to write the corrections box.

One question to ask. "What should the box say?" Then: "Would starting the chart at zero have hidden a real fall?" Let them argue both sides.

One thing to do at the table. Draw two bars for two numbers that are close together, such as the ages of two grown-ups at the table. Draw them once from zero and once starting just below the smaller number. Ask everyone which picture tells the truth about the difference.

The grown-up layer. The case separates true numbers from a misleading picture of them, and a clarification from a retraction. It gives the young reporter his strongest point, that a zero-based chart can make a real change invisible, and gives the librarian his: a picture can do harm that its numbers do not. The habits it models are going to the original log, looking at where a chart's axis begins, and dividing by the number of people before comparing two years.

The case leaves the line blank on purpose. Ask for it before you offer yours.