The Box Plot

A one-page box plot — the box-and-whisker chart — the one picture that compares whole distributions instead of the averages that hide inside them. Paste a raw list of readings for each group and the tool works out the five-number summary — the minimum, the lower quartile, the median, the upper quartile and the maximum — and draws the familiar box: it spans the middle half of the data, a line marks the median, and whiskers reach to the furthest points still inside one-and-a-half times the interquartile range. Anything past that is drawn as an outlier ring. Because every group shares one horizontal axis you can read four things at a glance that a single number can't tell you: where the typical value sits (the median line), how spread out the group is (the box width), whether it leans high or low (a median sitting off-centre in its box), and which readings are unusual (the rings). It's the right chart the moment "what's the average?" stops being enough — test scores across classes, response times across servers, prices across stores, delivery times across days, salaries across roles. Unlike the other charts here it takes the raw observations, not pre-totalled figures, so you can drop in a column of numbers straight from a spreadsheet. List a label and its list of numbers per line and the boxes, whiskers and outliers all compute themselves; leave it blank to print a bare axis and sketch by hand.

• Landscape
• 1 page
• Tracking

Best for: Comparing the spread and skew of several groups from their raw readings — test scores by class, response times by server, prices by store — as boxes with a median line, whiskers and outlier rings on one shared axis.