Direct labeling

A legend is an index. It tells the reader that this colour means that name, and leaves the matching up as an exercise. Direct labeling deletes the index by writing each name where its data already is, usually at the end of a line, sometimes on the mark itself. The information is identical. What changes is the number of times the eye has to leave the plot, and for a three-series chart that number goes from three to zero.

The cost of a legend is easy to underestimate because it is paid in working memory rather than in pixels. To read a legended line chart the reader looks at a line, carries a colour across to the key, finds the row, carries a name back, and hopes the line is still the one they left. Claus Wilke’s argument for redundant coding is the same argument one step further: colour is a weak channel to hang identity on, so give the series a second cue. Direct labeling is the strongest version of that cue, because the second channel is position, and position is the one thing the reader was already looking at. It also survives the failure modes colour does not: a printed page, a screenshot pasted into a document, a color vision deficiency that collapses two of the three hues.

It is a trade, not an upgrade, and the conditions are specific. Direct labeling needs room at the ends of the lines and ends that are far enough apart to label separately, which is why it is a pattern for three or four series and not for twelve. When the lines finish bunched together, or the plot has no right margin to give, or the series names are long enough to become the chart, the key earns its place back. Small multiples are the usual escape hatch when the count is too high for either: give each series its own panel and the panel title becomes the direct label.

The pairing to keep straight is with chart legend. A legend is a lookup you consult, a separate block that maps encoding to meaning and can be shared by several charts; direct labeling puts the name where the eye already is, and it can only exist once per mark. Read the other way: the legend is the component, direct labeling is the decision to do without it. Both sit inside chart, and both are downstream of categorical palette, which is where the colours being named come from in the first place. Direct labeling does not make the palette unnecessary, but it does lower the stakes: once every line says its own name, the palette only has to separate the lines, not identify them.