Legend

A legend is the key on a chart that maps each colour, pattern, or symbol to the data series or category it stands for.

The moment a chart shows more than one thing — two lines, a stack of coloured segments, several kinds of points — the reader needs to know which is which. The legend supplies that, pairing a small colour swatch or marker with a text label. It turns an otherwise ambiguous picture into something self-explanatory, so the chart can be understood without a separate caption.

How a legend works

A legend lists each data series or category alongside the visual cue used to draw it: a green square for one series, a blue one for another. It usually sits in a corner, along the top, or beside the plot. A well-built legend uses colours that are easy to tell apart and lists items in a sensible order — often matching the order the series appear in the chart. On a single-series chart no legend is needed, because there is nothing to distinguish.

A concrete example

Imagine a line chart comparing monthly visits to three sections of a site, drawn as a green, a blue, and a purple line. The legend reads: green = Blog, blue = Docs, purple = Pricing. With it, a reader who sees the top line is green immediately knows the blog drew the most visits. Strip the legend away and the three coloured lines become a puzzle — the data is intact, but its meaning is lost.

Related terms

Each entry in a legend names a data series, which is itself a collection of data points. Legends pair with the chart's axes to make a multi-series plot readable. Multi-series legends are common in the line chart guide.