Chart legend

A legend is a translation table for an encoding. The chart has turned each series into a colour or a symbol, and the legend is where that substitution is written down, so a reader who sees a violet band can find out that violet means trade. That makes it useful and expensive at the same time: every reading costs a trip out of the plot, into the key, and back to the mark the reader was looking at, with the mapping held in working memory across the round trip. Two series survive that easily. Seven do not, which is why a chart with a crowded legend is usually a chart that should have been split.

Placement is a choice about that round trip, and the shorter the trip the better the legend works. Above the plot puts the key on the reading path before the marks are looked at; beside it, in a column, matches the way a stacked bar or an area chart is read from the top down, and letting the order of the entries match the stacking order is what turns the legend into a map rather than a list. Inline, in the title line, works when there are only two or three series and the sentence can carry them. An interactive legend does more than translate: clicking an entry filters, isolates or hides that series, which is genuinely useful on a busy plot and needs its state shown on the entry itself, since a legend that quietly stops matching the plot is worse than none.

A legend keyed by colour alone fails any reader with a colour vision deficiency, and it fails everybody in greyscale print, so the use of colour rule applies here with full force: give each series a second cue, a fill pattern, a marker shape, a dash style, and repeat that cue in the key. Choosing the hues themselves is the job of a categorical palette, which is built so that no series reads as larger or later than another; picking them by eye tends to produce one colour that shouts and two that are the same.

The strongest argument against a legend is direct labeling: put each name at the end of its own line or inside its own band and the indirection disappears, because the reader never has to hold a mapping at all. The strongest argument for a legend is what direct labeling cannot do, which is name series whose marks are too small, too crowded, or too intertwined to hold a label of their own, and to give a stacked chart one ordered key instead of five repeated sets of names. Reach for labels first and keep the legend for the plots where labels will not fit; Carbon’s legend guidance treats it the same way, as apparatus to be earned rather than furniture to be added.

One caution about the word. HTML spends legend on something else entirely: the first child of a fieldset, which names a group of form controls and has nothing to do with colour. Say chart legend or key when the distinction matters, because a ticket asking for “a legend” has meant both.